485BPOS

As filed with the Securities and Exchange Commission on February 17, 2016

Securities Act File No. 033-12112

Investment Company Act File No. 811-05032

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM N-1A

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933	x
Pre-Effective Amendment No.	¨
Post-Effective Amendment No. 53	x

and/or

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1940	x
Amendment No. 54	x

(Check appropriate box or boxes)

BARON INVESTMENT FUNDS TRUST
f/k/a BARON ASSET FUND

(Exact Name of Registrant as Specified in Charter)

767 Fifth Avenue, 49th Floor

New York, NY 10153

(Address of Principal Executive Offices) (Zip Code)

(Registrant’s Telephone Number, including Area Code): 212-583-2000

Patrick M. Patalino, General Counsel

c/o Baron Investment Funds Trust

767 Fifth Avenue, 49th Floor

New York, NY 10153

(Name and Address of Agent for Service)

It is proposed that this filing will become effective (check appropriate box):

x Immediately upon filing pursuant to paragraph (b) of Rule 485

¨ On (date) pursuant to paragraph (b)(1)(iii) of Rule 485

¨ 60 days after filing pursuant to paragraph (a)(1) of Rule 485

¨ On (date) pursuant to paragraph (a)(1) of Rule 485

¨ 75 days after filing pursuant to paragraph (a)(2) of Rule 485

¨ On (date) pursuant to paragraph (a)(2) of Rule 485

If appropriate, check the following box:

¨ This post-effective amendment designates a new effective date for a previously filed post-effective amendment.

EXPLANATORY NOTE

This Post-Effective Amendment No. 53 relates solely to the following Funds, each a series of Baron Investment Funds Trust:

Baron Asset Fund

Baron Growth Fund

Baron Small Cap Fund

Baron Opportunity Fund

Baron Fifth Avenue Growth Fund

Baron Discovery Fund

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933 (the “Securities Act”) and the Investment Company Act of 1940, the Registrant certifies that it meets all of the requirements for effectiveness of this Registration Statement under Rule 485(b) under the Securities Act and has duly caused this Post-Effective Amendment No. 53 to the Registration Statement on Form N-1A to be signed on its behalf by the undersigned, thereunto duly authorized in the City of New York, and the State of New York, on this 17^th day of February, 2016.

BARON INVESTMENT FUNDS TRUST
By:	/s/ Ronald Baron
	Ronald Baron
	Chief Executive Officer

Pursuant to the requirements of the Securities Act of 1933, this Post-Effective Amendment No. 53 to the Registration Statement has been signed below by the following persons in the capacities and on the dates indicated.

SIGNATURES	TITLE	DATE
/s/ Ronald Baron Ronald Baron	Chief Executive Officer and Trustee	February 17, 2016
/s/ Linda S. Martinson Linda S. Martinson	Chairman, President, Chief Operating Officer and Trustee	February 17, 2016
/*s/ Norman S. Edelcup Norman S. Edelcup	Trustee	February 17, 2016
/s/ Patrick M. Patalino Patrick M. Patalino	Vice President, General Counsel and Secretary	February 17, 2016
/*s/ Harold Milner Harold Milner	Trustee	February 17, 2016
/*s/ Raymond Noveck Raymond Noveck	Trustee	February 17, 2016

/*s/ David A. Silverman David A. Silverman	Trustee	February 17, 2016
/s/ Peggy Wong Peggy Wong	Treasurer and Chief Financial Officer	February 17, 2016
/*s/ Alex Yemenidjian Alex Yemenidjian	Trustee	February 17, 2016
/*s/ Anita Rosenberg Anita Rosenberg	Trustee	February 17, 2016

By:	/s/ Patrick M. Patalino
	Patrick M. Patalino
	(Attorney-in-fact)

* Attorney-in-fact pursuant to a power of attorney previously filed.

Dated: February 17, 2016

Exhibit Index

Exhibit	Description
EX-101.INS	XBRL Instance Document
EX-101.SCH	XBRL Taxonomy Extension Schema Document
EX-101.CAL	XBRL Taxonomy Extension Calculation Linkbase
EX-101.DEF	XBRL Taxonomy Extension Definition Linkbase
EX-101.LAB	XBRL Taxonomy Extension Labels Linkbase
EX-101.PRE	XBRL Taxonomy Extension Presentation Linkbase

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Document Type	dei_DocumentType	485BPOS
Document Period End Date	dei_DocumentPeriodEndDate	Sep. 30, 2015
Registrant Name	dei_EntityRegistrantName	BARON INVESTMENT FUNDS TRUST (f/k/a BARON ASSET FUND)
Central Index Key	dei_EntityCentralIndexKey	0000810902
Amendment Flag	dei_AmendmentFlag	false
Document Creation Date	dei_DocumentCreationDate	Jan. 28, 2016
Document Effective Date	dei_DocumentEffectiveDate	Jan. 28, 2016
Prospectus Date	rr_ProspectusDate	Jan. 28, 2016

XML

BARON ASSET FUND

Baron Asset Fund

Investment Goal

The investment goal of Baron Asset Fund (the “Fund”) is capital appreciation through long-term investments primarily in securities of mid-sized companies with undervalued assets or favorable growth prospects.

Fees and Expenses of the Fund

The table below describes the fees and expenses that you would pay if you bought and held shares of the Fund.

Annual Fund Operating Expenses
(Expenses that you pay each year as a percentage of the value of your investment)

Annual Fund Operating Expenses - BARON ASSET FUND		Management Fee	Distribution (12b-1) Fee	Other Expenses	Total Annual Fund Operating Expenses
Retail Shares	[1]	1.00%	0.25%	0.06%	1.31%
Institutional Shares	[1]	1.00%	none	0.04%	1.04%
R6 Shares	[2]	1.00%	none	0.04%	1.04%

Example

This example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The example also assumes that your investment has a 5% return each year, and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example - BARON ASSET FUND - USD ($)	1	3	5	10
Retail Shares	133	415	718	1,579
Institutional Shares	106	331	574	1,271
R6 Shares	106	331	574	1,271

Portfolio Turnover.

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes for Fund shareholders. These costs, which are not reflected in Total Annual Fund Operating Expenses or in the example, affect the Fund’s performance. During the most recent fiscal year ended September 30, 2015, the Fund’s portfolio turnover rate was 13.53% of the average value of its portfolio.

Investments, Risks, and Performance

Principal Investment Strategies of the Fund

The Fund invests for the long term primarily in equity securities in the form of common stock of mid-sized growth companies. BAMCO, Inc. (“BAMCO” or the “Adviser”) defines mid-sized companies as those, at the time of purchase, with market capitalizations above $2.5 billion or the smallest market cap stock in the Russell Midcap Growth Index at reconstitution, whichever is larger, and below the largest market cap stock in the Russell Midcap Growth Index at reconstitution. At the last reconstitution by Russell on June 30, 2015, the market cap range of stocks in the Russell Midcap Growth Index was $2.18 billion and $28.09 billion. The Adviser seeks to invest in businesses it believes have significant opportunities for growth, sustainable competitive advantages, exceptional management, and an attractive valuation. Because of its long-term approach, the Fund could have a significant percentage of its assets invested in securities that have appreciated beyond their market capitalizations at the time of the Fund’s investment.

Principal Risks of Investing in the Fund

General Stock Market.  Investing in the stock market is risky because securities fluctuate in value. These fluctuations may be due to political, economic or general market circumstances. Other factors may affect a single company or industry but not the broader market. Because the values of securities fluctuate, when you sell your investment in the Fund, you may lose money. Current and future portfolio holdings in the Fund are subject to risk.

Mid-Sized Companies.  The Adviser believes there is more potential for capital appreciation in mid-sized companies, but there also may be more risk. Securities of mid-sized companies may not be well known to most investors, and the securities may be less actively traded than those of large businesses. The securities of mid-sized companies may fluctuate in price more widely than the stock market generally, and they may be more difficult to sell during market downturns. Mid-sized companies rely more on the skills of management and on their continued tenure. Investing in mid-sized companies requires a long-term outlook and may require shareholders to assume more risk and to have more patience than investing in the securities of larger, more established companies.

Growth Investing.  Growth stocks can react differently to issuer, political, market and economic developments than the market as a whole and other types of stocks. Growth stocks tend to be more expensive relative to their earnings or assets compared to other types of stocks. As a result, growth stocks tend to be sensitive to changes in their earnings and more volatile than other types of stocks.

Performance

The following bar chart and table provide some indication of the risks of investing in the Fund (Retail Shares) by showing changes in the Fund’s performance from year to year and by showing how the Fund’s average annual returns for 1, 5 and 10 years compare with those of a broad measure of market performance. The Fund’s past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. Updated performance information is available online at www.BaronFunds.com/performance or by calling 1-800-99BARON (1-800-992-2766).

Year by Year Total Return (%) as of December 31 of Each Year (Retail Shares)

Best Quarter:  06/30/09:  20.73%

Worst Quarter:  12/31/08: (27.51%)

Average Annual Total Returns (for periods ended 12/31/15)

The table below shows the Fund's Retail Shares' annual returns and long-term performance (before and after taxes) and the change in value of broad-based market indexes over various periods ended December 31, 2015. The table also shows the average annual returns of the Fund's Institutional Shares, but it does not show after-tax returns.

After-tax returns are calculated using the highest individual federal marginal income tax rate in effect at the time of each distribution and assumed sale, but they do not include the impact of state and local taxes.

Your actual after-tax returns depend on your own tax situation and may differ from those shown. After-tax returns reflect past tax effects and are not predictive of future tax effects. After-tax returns are not relevant to investors who hold their Fund's shares in a tax-deferred account (including a 401(k) or IRA or Coverdell account), or to investors that are tax-exempt.

Average Annual Total Returns for the periods ended December 31, 2015

Average Annual Total Returns - BARON ASSET FUND		1 year	5 years	10 years	Since Inception		Inception date
Retail Shares		(0.08%)	11.20%	7.35%	11.14%		Jun. 12, 1987
Retail Shares | Return after taxes on distributions		(2.53%)	9.14%	6.04%	10.15%		Jun. 12, 1987
Retail Shares | Return after taxes on distributions and sale of Fund shares		1.98%	8.91%	5.93%	9.76%		Jun. 12, 1987
Institutional Shares	[1]	0.20%	11.50%	7.54%	11.21%		May 29, 2009
Russell Midcap® Growth Index (reflects no deduction for fees, expenses or taxes)		(0.20%)	11.54%	8.16%	9.75%	[2]
S&P 500 Index (reflects no deduction for fees, expenses or taxes)		1.38%	12.57%	7.31%	9.37%

The Fund’s R6 Shares are newly issued and therefore have no performance as of the date of this prospectus. The performance of R6 Shares will differ from that shown above only to the extent that the classes do not have the same expenses.

The Russell Midcap^® Growth Index is an unmanaged index of mid-cap growth companies. The S&P 500 Index is an unmanaged index of larger-cap companies.

[1] Based on fiscal year ended September 30, 2015.

[2] Because R6 Shares are newly issued, the Other Expenses are based on estimated amounts for the current fiscal year.

[1] Performance for the Institutional Shares prior to May 29, 2009 is based on the performance of the Retail Shares, which have a distribution fee. The Institutional Shares do not have a distribution fee. If the annual returns for the Institutional Shares prior to May 29, 2009 did not reflect this fee, the returns would be higher.

[2] For the period June 30, 1987 to December 31, 2015.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

BARON INVESTMENT FUNDS TRUST (f/k/a BARON ASSET FUND)

Prospectus Date

rr_ProspectusDate

Jan. 28, 2016

BARON ASSET FUND

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Baron Asset Fund

Objective [Heading]

rr_ObjectiveHeading

Investment Goal

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The investment goal of Baron Asset Fund (the “Fund”) is capital appreciation through long-term investments primarily in securities of mid-sized companies with undervalued assets or favorable growth prospects.

Expense [Heading]

rr_ExpenseHeading

Fees and Expenses of the Fund

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The table below describes the fees and expenses that you would pay if you bought and held shares of the Fund.

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

Annual Fund Operating Expenses
(Expenses that you pay each year as a percentage of the value of your investment)

Portfolio Turnover [Heading]

rr_PortfolioTurnoverHeading

Portfolio Turnover.

Portfolio Turnover [Text Block]

rr_PortfolioTurnoverTextBlock

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes for Fund shareholders. These costs, which are not reflected in Total Annual Fund Operating Expenses or in the example, affect the Fund’s performance. During the most recent fiscal year ended September 30, 2015, the Fund’s portfolio turnover rate was 13.53% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

13.53%

Other Expenses, New Fund, Based on Estimates [Text]

rr_OtherExpensesNewFundBasedOnEstimates

Because R6 Shares are newly issued, the Other Expenses are based on estimated amounts for the current fiscal year.

Expense Example [Heading]

rr_ExpenseExampleHeading

Example

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The example also assumes that your investment has a 5% return each year, and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Strategy [Heading]

rr_StrategyHeading

Investments, Risks, and Performance

Principal Investment Strategies of the Fund

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

The Fund invests for the long term primarily in equity securities in the form of common stock of mid-sized growth companies. BAMCO, Inc. (“BAMCO” or the “Adviser”) defines mid-sized companies as those, at the time of purchase, with market capitalizations above $2.5 billion or the smallest market cap stock in the Russell Midcap Growth Index at reconstitution, whichever is larger, and below the largest market cap stock in the Russell Midcap Growth Index at reconstitution. At the last reconstitution by Russell on June 30, 2015, the market cap range of stocks in the Russell Midcap Growth Index was $2.18 billion and $28.09 billion. The Adviser seeks to invest in businesses it believes have significant opportunities for growth, sustainable competitive advantages, exceptional management, and an attractive valuation. Because of its long-term approach, the Fund could have a significant percentage of its assets invested in securities that have appreciated beyond their market capitalizations at the time of the Fund’s investment.

Risk [Heading]

rr_RiskHeading

Principal Risks of Investing in the Fund

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

General Stock Market.  Investing in the stock market is risky because securities fluctuate in value. These fluctuations may be due to political, economic or general market circumstances. Other factors may affect a single company or industry but not the broader market. Because the values of securities fluctuate, when you sell your investment in the Fund, you may lose money. Current and future portfolio holdings in the Fund are subject to risk.

Mid-Sized Companies.  The Adviser believes there is more potential for capital appreciation in mid-sized companies, but there also may be more risk. Securities of mid-sized companies may not be well known to most investors, and the securities may be less actively traded than those of large businesses. The securities of mid-sized companies may fluctuate in price more widely than the stock market generally, and they may be more difficult to sell during market downturns. Mid-sized companies rely more on the skills of management and on their continued tenure. Investing in mid-sized companies requires a long-term outlook and may require shareholders to assume more risk and to have more patience than investing in the securities of larger, more established companies.

Growth Investing.  Growth stocks can react differently to issuer, political, market and economic developments than the market as a whole and other types of stocks. Growth stocks tend to be more expensive relative to their earnings or assets compared to other types of stocks. As a result, growth stocks tend to be sensitive to changes in their earnings and more volatile than other types of stocks.

Risk Lose Money [Text]

rr_RiskLoseMoney

Because the values of securities fluctuate, when you sell your investment in the Fund, you may lose money.

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

Performance

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following bar chart and table provide some indication of the risks of investing in the Fund (Retail Shares) by showing changes in the Fund’s performance from year to year and by showing how the Fund’s average annual returns for 1, 5 and 10 years compare with those of a broad measure of market performance. The Fund’s past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. Updated performance information is available online at www.BaronFunds.com/performance or by calling 1-800-99BARON (1-800-992-2766).

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

The following bar chart and table provide some indication of the risks of investing in the Fund (Retail Shares) by showing changes in the Fund’s performance from year to year and by showing how the Fund’s average annual returns for 1, 5 and 10 years compare with those of a broad measure of market performance.

Performance One Year or Less [Text]

rr_PerformanceOneYearOrLess

The Fund’s R6 Shares are newly issued and therefore have no performance as of the date of this prospectus.

Performance Additional Market Index [Text]

rr_PerformanceAdditionalMarketIndex

The Russell Midcap^® Growth Index is an unmanaged index of mid-cap growth companies. The S&P 500 Index is an unmanaged index of larger-cap companies.

Performance Availability Phone [Text]

rr_PerformanceAvailabilityPhone

1-800-99BARON (1-800-992-2766)

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

www.BaronFunds.com/performance

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

The Fund’s past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future.

Bar Chart [Heading]

rr_BarChartHeading

Year by Year Total Return (%) as of December 31 of Each Year (Retail Shares)

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

Best Quarter:  06/30/09:  20.73%

Worst Quarter:  12/31/08: (27.51%)

Performance Table Heading

rr_PerformanceTableHeading

Average Annual Total Returns (for periods ended 12/31/15)

Performance Table Uses Highest Federal Rate

rr_PerformanceTableUsesHighestFederalRate

After-tax returns are calculated using the highest individual federal marginal income tax rate in effect at the time of each distribution and assumed sale, but they do not include the impact of state and local taxes.

Performance Table Not Relevant to Tax Deferred

rr_PerformanceTableNotRelevantToTaxDeferred

After-tax returns are not relevant to investors who hold their Fund’s shares in a tax-deferred account (including a 401(k) or IRA or Coverdell account), or to investors that are tax-exempt.

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

The table below shows the Fund's Retail Shares' annual returns and long-term performance (before and after taxes) and the change in value of broad-based market indexes over various periods ended December 31, 2015. The table also shows the average annual returns of the Fund's Institutional Shares, but it does not show after-tax returns.

After-tax returns are calculated using the highest individual federal marginal income tax rate in effect at the time of each distribution and assumed sale, but they do not include the impact of state and local taxes.

Your actual after-tax returns depend on your own tax situation and may differ from those shown. After-tax returns reflect past tax effects and are not predictive of future tax effects. After-tax returns are not relevant to investors who hold their Fund's shares in a tax-deferred account (including a 401(k) or IRA or Coverdell account), or to investors that are tax-exempt.

Performance Table Closing [Text Block]

rr_PerformanceTableClosingTextBlock

The Fund’s R6 Shares are newly issued and therefore have no performance as of the date of this prospectus. The performance of R6 Shares will differ from that shown above only to the extent that the classes do not have the same expenses.

The Russell Midcap^® Growth Index is an unmanaged index of mid-cap growth companies. The S&P 500 Index is an unmanaged index of larger-cap companies.

Caption

rr_AverageAnnualReturnCaption

Average Annual Total Returns for the periods ended December 31, 2015

BARON ASSET FUND | Retail Shares

Risk/Return:

rr_RiskReturnAbstract

Management Fee

rr_ManagementFeesOverAssets

1.00%

^[1]

Distribution (12b-1) Fee

rr_DistributionAndService12b1FeesOverAssets

0.25%

^[1]

Other Expenses

rr_OtherExpensesOverAssets

0.06%

^[1]

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.31%

^[1]

1

rr_ExpenseExampleYear01

$ 133

3

rr_ExpenseExampleYear03

415

5

rr_ExpenseExampleYear05

718

10

rr_ExpenseExampleYear10

$ 1,579

2006

rr_AnnualReturn2006

14.64%

2007

rr_AnnualReturn2007

10.13%

2008

rr_AnnualReturn2008

(40.75%)

2009

rr_AnnualReturn2009

31.70%

2010

rr_AnnualReturn2010

21.29%

2011

rr_AnnualReturn2011

(2.94%)

2012

rr_AnnualReturn2012

15.31%

2013

rr_AnnualReturn2013

38.88%

2014

rr_AnnualReturn2014

9.48%

2015

rr_AnnualReturn2015

(0.08%)

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Best Quarter:

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Jun. 30, 2009

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

20.73%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Worst Quarter:

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Dec. 31, 2008

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(27.51%)

1 year

rr_AverageAnnualReturnYear01

(0.08%)

5 years

rr_AverageAnnualReturnYear05

11.20%

10 years

rr_AverageAnnualReturnYear10

7.35%

Since Inception

rr_AverageAnnualReturnSinceInception

11.14%

Inception date

rr_AverageAnnualReturnInceptionDate

Jun. 12, 1987

BARON ASSET FUND | Institutional Shares

Risk/Return:

rr_RiskReturnAbstract

Management Fee

rr_ManagementFeesOverAssets

1.00%

^[1]

Distribution (12b-1) Fee

rr_DistributionAndService12b1FeesOverAssets

none

^[1]

Other Expenses

rr_OtherExpensesOverAssets

0.04%

^[1]

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.04%

^[1]

1

rr_ExpenseExampleYear01

$ 106

3

rr_ExpenseExampleYear03

331

5

rr_ExpenseExampleYear05

574

10

rr_ExpenseExampleYear10

$ 1,271

1 year

rr_AverageAnnualReturnYear01

0.20%

^[2]

5 years

rr_AverageAnnualReturnYear05

11.50%

^[2]

10 years

rr_AverageAnnualReturnYear10

7.54%

^[2]

Since Inception

rr_AverageAnnualReturnSinceInception

11.21%

^[2]

Inception date

rr_AverageAnnualReturnInceptionDate

May 29, 2009

^[2]

BARON ASSET FUND | R6 Shares

Risk/Return:

rr_RiskReturnAbstract

Management Fee

rr_ManagementFeesOverAssets

1.00%

^[3]

Distribution (12b-1) Fee

rr_DistributionAndService12b1FeesOverAssets

none

^[3]

Other Expenses

rr_OtherExpensesOverAssets

0.04%

^[3]

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.04%

^[3]

1

rr_ExpenseExampleYear01

$ 106

3

rr_ExpenseExampleYear03

331

5

rr_ExpenseExampleYear05

574

10

rr_ExpenseExampleYear10

$ 1,271

BARON ASSET FUND | Return after taxes on distributions | Retail Shares

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

(2.53%)

5 years

rr_AverageAnnualReturnYear05

9.14%

10 years

rr_AverageAnnualReturnYear10

6.04%

Since Inception

rr_AverageAnnualReturnSinceInception

10.15%

Inception date

rr_AverageAnnualReturnInceptionDate

Jun. 12, 1987

BARON ASSET FUND | Return after taxes on distributions and sale of Fund shares | Retail Shares

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

1.98%

5 years

rr_AverageAnnualReturnYear05

8.91%

10 years

rr_AverageAnnualReturnYear10

5.93%

Since Inception

rr_AverageAnnualReturnSinceInception

9.76%

Inception date

rr_AverageAnnualReturnInceptionDate

Jun. 12, 1987

BARON ASSET FUND | Russell Midcap® Growth Index (reflects no deduction for fees, expenses or taxes)

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

(0.20%)

5 years

rr_AverageAnnualReturnYear05

11.54%

10 years

rr_AverageAnnualReturnYear10

8.16%

Since Inception

rr_AverageAnnualReturnSinceInception

9.75%

^[4]

BARON ASSET FUND | S&P 500 Index (reflects no deduction for fees, expenses or taxes)

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

1.38%

5 years

rr_AverageAnnualReturnYear05

12.57%

10 years

rr_AverageAnnualReturnYear10

7.31%

Since Inception

rr_AverageAnnualReturnSinceInception

9.37%

[1] Based on fiscal year ended September 30, 2015.

[2] Performance for the Institutional Shares prior to May 29, 2009 is based on the performance of the Retail Shares, which have a distribution fee. The Institutional Shares do not have a distribution fee. If the annual returns for the Institutional Shares prior to May 29, 2009 did not reflect this fee, the returns would be higher.

[3] Because R6 Shares are newly issued, the Other Expenses are based on estimated amounts for the current fiscal year.

[4] For the period June 30, 1987 to December 31, 2015.

XML

BARON GROWTH FUND

Baron Growth Fund

Investment Goal

The investment goal of Baron Growth Fund (the “Fund”) is capital appreciation through long-term investments primarily in securities of small-sized growth companies.

Fees and Expenses of the Fund

The table below describes the fees and expenses that you would pay if you bought and held shares of the Fund.

Annual Fund Operating Expenses
(Expenses that you pay each year as a percentage of the value of your investment)

Annual Fund Operating Expenses - BARON GROWTH FUND		Management Fee	Distribution (12b-1) Fee	Other Expenses	Total Annual Fund Operating Expenses
Retail Shares	[1]	1.00%	0.25%	0.04%	1.29%
Institutional Shares	[1]	1.00%	none	0.04%	1.04%
R6 Shares	[2]	1.00%	none	0.04%	1.04%

Example

This example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example - BARON GROWTH FUND - USD ($)	1	3	5	10
Retail Shares	131	409	708	1,556
Institutional Shares	106	331	574	1,271
R6 Shares	106	331	574	1,271

Portfolio Turnover.

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes for Fund shareholders. These costs, which are not reflected in Total Annual Fund Operating Expenses or in the example, affect the Fund’s performance. During the most recent fiscal year ended September 30, 2015, the Fund’s portfolio turnover rate was 6.95% of the average value of its portfolio.

Investments, Risks, and Performance

Principal Investment Strategies of the Fund

The Fund invests for the long term primarily in equity securities in the form of common stock of small-sized growth companies. BAMCO, Inc. (“BAMCO” or the “Adviser”) defines small-sized companies as those, at the time of purchase, with market capitalizations up to the largest market cap stock in the Russell 2000 Growth Index at reconstitution, or companies with market capitalizations up to $2.5 billion, whichever is larger. At the last reconstitution by Russell on June 30, 2015, the largest market cap stock in the Russell 2000 Growth Index was $4.71 billion. The Adviser seeks to invest in businesses it believes have significant opportunities for growth, sustainable competitive advantages, exceptional management, and an attractive valuation. Because of its long-term approach, the Fund could have a significant percentage of its assets invested in securities that have appreciated beyond their market capitalizations at the time of the Fund’s investment.

Principal Risks of Investing in the Fund

General Stock Market. Investing in the stock market is risky because securities fluctuate in value. These fluctuations may be due to political, economic or general market circumstances. Other factors may affect a single company or industry but not the broader market. Because the values of securities fluctuate, when you sell your investment in the Fund, you may lose money. Current and future portfolio holdings in the Fund are subject to risk.

Small-Sized Companies.  The Adviser believes there is more potential for capital appreciation in small-sized companies, but there also may be more risk. Securities of small-sized companies may not be well known to most investors, and the securities may be less actively traded than those of large businesses. The securities of small-sized companies may fluctuate in price more widely than the stock market generally, and they may be more difficult to sell during market downturns. Small-sized companies rely more on the skills of management and on their continued tenure. Investing in small-sized companies requires a long-term outlook and may require shareholders to assume more risk and to have more patience than investing in the securities of larger, more established companies.

Growth Investing.  Growth stocks can react differently to issuer, political, market and economic developments than the market as a whole and other types of stocks. Growth stocks tend to be more expensive relative to their earnings or assets compared to other types of stocks. As a result, growth stocks tend to be sensitive to changes in their earnings and more volatile than other types of stocks.

Performance

The following bar chart and table provide some indication of the risks of investing in the Fund (Retail Shares) by showing changes in the Fund’s performance from year to year and by showing how the Fund’s average annual returns for 1, 5 and 10 years compare with those of a broad measure of market performance. The Fund’s past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. Updated performance information is available online at www.BaronFunds.com/performance or by calling 1-800-99BARON (1-800-992-2766).

Year by Year Total Return (%) as of December 31 of Each Year (Retail Shares)

Best Quarter:  06/30/09:  20.76%

Worst Quarter:  12/31/08: (26.10%)

Average Annual Total Returns (for periods ended 12/31/15)

The table below shows the Fund’s Retail Shares’ annual returns and long-term performance (before and after taxes) and the change in value of broad-based market indexes over various periods ended December 31, 2015. The table also shows the average annual returns of the Fund’s Institutional Shares, but it does not show after-tax returns.

After-tax returns are calculated using the highest individual federal marginal income tax rate in effect at the time of each distribution and assumed sale, but they do not include the impact of state and local taxes.

Your actual after-tax returns depend on your own tax situation and may differ from those shown. After-tax returns reflect past tax effects and are not predictive of future tax effects. After-tax returns are not relevant to investors who hold their Fund’s shares in a tax-deferred account (including a 401(k) or IRA or Coverdell account), or to investors that are tax-exempt.

Average Annual Total Returns for the periods ended December 31, 2015

Average Annual Total Returns - BARON GROWTH FUND		1 year	5 years	10 years	Since Inception	Inception date
Retail Shares		(4.31%)	10.25%	7.34%	12.77%	Dec. 31, 1994
Retail Shares | Return after taxes on distributions		(6.26%)	9.06%	6.60%	12.07%	Dec. 31, 1994
Retail Shares | Return after taxes on distributions and sale of Fund shares		(0.79%)	8.23%	6.01%	11.26%	Dec. 31, 1994
Institutional Shares	[1]	(4.06%)	10.54%	7.52%	12.87%	May 29, 2009
Russell 2000® Growth Index (reflects no deduction for fees, expenses or taxes)		(1.38%)	10.67%	7.95%	7.37%
S&P 500 Index (reflects no deduction for fees, expenses or taxes)		1.38%	12.57%	7.31%	9.43%

The Fund’s R6 Shares are newly issued and therefore have no performance as of the date of this prospectus. The performance of R6 Shares will differ from that shown above only to the extent that the classes do not have the same expenses.

The Russell 2000^® Growth Index is an unmanaged index of U.S. small-cap growth companies. The S&P 500 Index is an unmanaged index of larger-cap companies.

[1] Based on the fiscal year ended September 30, 2015.

[2] Because R6 Shares are newly issued, the Other Expenses are based on estimated amounts for the current fiscal year.

[1] Performance for the Institutional Shares prior to May 29, 2009 is based on the performance of the Retail Shares, which have a distribution fee. The Institutional Shares do not have a distribution fee. If the annual returns for the Institutional Shares prior to May 29, 2009 did not reflect this fee, the returns would be higher.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

BARON INVESTMENT FUNDS TRUST (f/k/a BARON ASSET FUND)

Prospectus Date

rr_ProspectusDate

Jan. 28, 2016

BARON GROWTH FUND

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Baron Growth Fund

Objective [Heading]

rr_ObjectiveHeading

Investment Goal

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The investment goal of Baron Growth Fund (the “Fund”) is capital appreciation through long-term investments primarily in securities of small-sized growth companies.

Expense [Heading]

rr_ExpenseHeading

Fees and Expenses of the Fund

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The table below describes the fees and expenses that you would pay if you bought and held shares of the Fund.

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

Annual Fund Operating Expenses
(Expenses that you pay each year as a percentage of the value of your investment)

Portfolio Turnover [Heading]

rr_PortfolioTurnoverHeading

Portfolio Turnover.

Portfolio Turnover [Text Block]

rr_PortfolioTurnoverTextBlock

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes for Fund shareholders. These costs, which are not reflected in Total Annual Fund Operating Expenses or in the example, affect the Fund’s performance. During the most recent fiscal year ended September 30, 2015, the Fund’s portfolio turnover rate was 6.95% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

6.95%

Other Expenses, New Fund, Based on Estimates [Text]

rr_OtherExpensesNewFundBasedOnEstimates

Because R6 Shares are newly issued, the Other Expenses are based on estimated amounts for the current fiscal year.

Expense Example [Heading]

rr_ExpenseExampleHeading

Example

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Strategy [Heading]

rr_StrategyHeading

Investments, Risks, and Performance

Principal Investment Strategies of the Fund

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

The Fund invests for the long term primarily in equity securities in the form of common stock of small-sized growth companies. BAMCO, Inc. (“BAMCO” or the “Adviser”) defines small-sized companies as those, at the time of purchase, with market capitalizations up to the largest market cap stock in the Russell 2000 Growth Index at reconstitution, or companies with market capitalizations up to $2.5 billion, whichever is larger. At the last reconstitution by Russell on June 30, 2015, the largest market cap stock in the Russell 2000 Growth Index was $4.71 billion. The Adviser seeks to invest in businesses it believes have significant opportunities for growth, sustainable competitive advantages, exceptional management, and an attractive valuation. Because of its long-term approach, the Fund could have a significant percentage of its assets invested in securities that have appreciated beyond their market capitalizations at the time of the Fund’s investment.

Risk [Heading]

rr_RiskHeading

Principal Risks of Investing in the Fund

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

General Stock Market. Investing in the stock market is risky because securities fluctuate in value. These fluctuations may be due to political, economic or general market circumstances. Other factors may affect a single company or industry but not the broader market. Because the values of securities fluctuate, when you sell your investment in the Fund, you may lose money. Current and future portfolio holdings in the Fund are subject to risk.

Small-Sized Companies.  The Adviser believes there is more potential for capital appreciation in small-sized companies, but there also may be more risk. Securities of small-sized companies may not be well known to most investors, and the securities may be less actively traded than those of large businesses. The securities of small-sized companies may fluctuate in price more widely than the stock market generally, and they may be more difficult to sell during market downturns. Small-sized companies rely more on the skills of management and on their continued tenure. Investing in small-sized companies requires a long-term outlook and may require shareholders to assume more risk and to have more patience than investing in the securities of larger, more established companies.

Growth Investing.  Growth stocks can react differently to issuer, political, market and economic developments than the market as a whole and other types of stocks. Growth stocks tend to be more expensive relative to their earnings or assets compared to other types of stocks. As a result, growth stocks tend to be sensitive to changes in their earnings and more volatile than other types of stocks.

Risk Lose Money [Text]

rr_RiskLoseMoney

Because the values of securities fluctuate, when you sell your investment in the Fund, you may lose money.

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

Performance

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following bar chart and table provide some indication of the risks of investing in the Fund (Retail Shares) by showing changes in the Fund’s performance from year to year and by showing how the Fund’s average annual returns for 1, 5 and 10 years compare with those of a broad measure of market performance. The Fund’s past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. Updated performance information is available online at www.BaronFunds.com/performance or by calling 1-800-99BARON (1-800-992-2766).

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

The following bar chart and table provide some indication of the risks of investing in the Fund (Retail Shares) by showing changes in the Fund’s performance from year to year and by showing how the Fund’s average annual returns for 1, 5 and 10 years compare with those of a broad measure of market performance.

Performance One Year or Less [Text]

rr_PerformanceOneYearOrLess

The Fund’s R6 Shares are newly issued and therefore have no performance as of the date of this prospectus.

Performance Additional Market Index [Text]

rr_PerformanceAdditionalMarketIndex

The Russell 2000^® Growth Index is an unmanaged index of U.S. small-cap growth companies. The S&P 500 Index is an unmanaged index of larger-cap companies.

Performance Availability Phone [Text]

rr_PerformanceAvailabilityPhone

1-800-99BARON (1-800-992-2766)

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

www.BaronFunds.com/performance

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

The Fund’s past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future.

Bar Chart [Heading]

rr_BarChartHeading

Year by Year Total Return (%) as of December 31 of Each Year (Retail Shares)

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

Best Quarter:  06/30/09:  20.76%

Worst Quarter:  12/31/08: (26.10%)

Performance Table Heading

rr_PerformanceTableHeading

Average Annual Total Returns (for periods ended 12/31/15)

Performance Table Uses Highest Federal Rate

rr_PerformanceTableUsesHighestFederalRate

After-tax returns are calculated using the highest individual federal marginal income tax rate in effect at the time of each distribution and assumed sale, but they do not include the impact of state and local taxes.

Performance Table Not Relevant to Tax Deferred

rr_PerformanceTableNotRelevantToTaxDeferred

After-tax returns are not relevant to investors who hold their Fund’s shares in a tax-deferred account (including a 401(k) or IRA or Coverdell account), or to investors that are tax-exempt.

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

The table below shows the Fund’s Retail Shares’ annual returns and long-term performance (before and after taxes) and the change in value of broad-based market indexes over various periods ended December 31, 2015. The table also shows the average annual returns of the Fund’s Institutional Shares, but it does not show after-tax returns.

After-tax returns are calculated using the highest individual federal marginal income tax rate in effect at the time of each distribution and assumed sale, but they do not include the impact of state and local taxes.

Your actual after-tax returns depend on your own tax situation and may differ from those shown. After-tax returns reflect past tax effects and are not predictive of future tax effects. After-tax returns are not relevant to investors who hold their Fund’s shares in a tax-deferred account (including a 401(k) or IRA or Coverdell account), or to investors that are tax-exempt.

Performance Table Closing [Text Block]

rr_PerformanceTableClosingTextBlock

The Fund’s R6 Shares are newly issued and therefore have no performance as of the date of this prospectus. The performance of R6 Shares will differ from that shown above only to the extent that the classes do not have the same expenses.

The Russell 2000^® Growth Index is an unmanaged index of U.S. small-cap growth companies. The S&P 500 Index is an unmanaged index of larger-cap companies.

Caption

rr_AverageAnnualReturnCaption

Average Annual Total Returns for the periods ended December 31, 2015

BARON GROWTH FUND | Retail Shares

Risk/Return:

rr_RiskReturnAbstract

Management Fee

rr_ManagementFeesOverAssets

1.00%

^[1]

Distribution (12b-1) Fee

rr_DistributionAndService12b1FeesOverAssets

0.25%

^[1]

Other Expenses

rr_OtherExpensesOverAssets

0.04%

^[1]

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.29%

^[1]

1

rr_ExpenseExampleYear01

$ 131

3

rr_ExpenseExampleYear03

409

5

rr_ExpenseExampleYear05

708

10

rr_ExpenseExampleYear10

$ 1,556

2006

rr_AnnualReturn2006

15.50%

2007

rr_AnnualReturn2007

6.59%

2008

rr_AnnualReturn2008

(39.18%)

2009

rr_AnnualReturn2009

34.24%

2010

rr_AnnualReturn2010

24.01%

2011

rr_AnnualReturn2011

1.24%

2012

rr_AnnualReturn2012

16.43%

2013

rr_AnnualReturn2013

38.32%

2014

rr_AnnualReturn2014

4.40%

2015

rr_AnnualReturn2015

(4.31%)

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Best Quarter:

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Jun. 30, 2009

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

20.76%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Worst Quarter:

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Dec. 31, 2008

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(26.10%)

1 year

rr_AverageAnnualReturnYear01

(4.31%)

5 years

rr_AverageAnnualReturnYear05

10.25%

10 years

rr_AverageAnnualReturnYear10

7.34%

Since Inception

rr_AverageAnnualReturnSinceInception

12.77%

Inception date

rr_AverageAnnualReturnInceptionDate

Dec. 31, 1994

BARON GROWTH FUND | Institutional Shares

Risk/Return:

rr_RiskReturnAbstract

Management Fee

rr_ManagementFeesOverAssets

1.00%

^[1]

Distribution (12b-1) Fee

rr_DistributionAndService12b1FeesOverAssets

none

^[1]

Other Expenses

rr_OtherExpensesOverAssets

0.04%

^[1]

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.04%

^[1]

1

rr_ExpenseExampleYear01

$ 106

3

rr_ExpenseExampleYear03

331

5

rr_ExpenseExampleYear05

574

10

rr_ExpenseExampleYear10

$ 1,271

1 year

rr_AverageAnnualReturnYear01

(4.06%)

^[2]

5 years

rr_AverageAnnualReturnYear05

10.54%

^[2]

10 years

rr_AverageAnnualReturnYear10

7.52%

^[2]

Since Inception

rr_AverageAnnualReturnSinceInception

12.87%

^[2]

Inception date

rr_AverageAnnualReturnInceptionDate

May 29, 2009

^[2]

BARON GROWTH FUND | R6 Shares

Risk/Return:

rr_RiskReturnAbstract

Management Fee

rr_ManagementFeesOverAssets

1.00%

^[3]

Distribution (12b-1) Fee

rr_DistributionAndService12b1FeesOverAssets

none

^[3]

Other Expenses

rr_OtherExpensesOverAssets

0.04%

^[3]

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.04%

^[3]

1

rr_ExpenseExampleYear01

$ 106

3

rr_ExpenseExampleYear03

331

5

rr_ExpenseExampleYear05

574

10

rr_ExpenseExampleYear10

$ 1,271

BARON GROWTH FUND | Return after taxes on distributions | Retail Shares

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

(6.26%)

5 years

rr_AverageAnnualReturnYear05

9.06%

10 years

rr_AverageAnnualReturnYear10

6.60%

Since Inception

rr_AverageAnnualReturnSinceInception

12.07%

Inception date

rr_AverageAnnualReturnInceptionDate

Dec. 31, 1994

BARON GROWTH FUND | Return after taxes on distributions and sale of Fund shares | Retail Shares

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

(0.79%)

5 years

rr_AverageAnnualReturnYear05

8.23%

10 years

rr_AverageAnnualReturnYear10

6.01%

Since Inception

rr_AverageAnnualReturnSinceInception

11.26%

Inception date

rr_AverageAnnualReturnInceptionDate

Dec. 31, 1994

BARON GROWTH FUND | Russell 2000® Growth Index (reflects no deduction for fees, expenses or taxes)

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

(1.38%)

5 years

rr_AverageAnnualReturnYear05

10.67%

10 years

rr_AverageAnnualReturnYear10

7.95%

Since Inception

rr_AverageAnnualReturnSinceInception

7.37%

BARON GROWTH FUND | S&P 500 Index (reflects no deduction for fees, expenses or taxes)

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

1.38%

5 years

rr_AverageAnnualReturnYear05

12.57%

10 years

rr_AverageAnnualReturnYear10

7.31%

Since Inception

rr_AverageAnnualReturnSinceInception

9.43%

[1] Based on the fiscal year ended September 30, 2015.

[2] Performance for the Institutional Shares prior to May 29, 2009 is based on the performance of the Retail Shares, which have a distribution fee. The Institutional Shares do not have a distribution fee. If the annual returns for the Institutional Shares prior to May 29, 2009 did not reflect this fee, the returns would be higher.

[3] Because R6 Shares are newly issued, the Other Expenses are based on estimated amounts for the current fiscal year.

XML

BARON SMALL CAP FUND

Baron Small Cap Fund

Investment Goal

The investment goal of Baron Small Cap Fund (the “Fund”) is capital appreciation through investments primarily in securities of small-sized growth companies.

Fees and Expenses of the Fund

The table below describes the fees and expenses that you would pay if you bought and held shares of the Fund.

Annual Fund Operating Expenses
(Expenses that you pay each year as a percentage of the value of your investment)

Annual Fund Operating Expenses - BARON SMALL CAP FUND		Management Fee	Distribution (12b-1) Fee	Other Expenses	Total Annual Fund Operating Expenses
Retail Shares	[1]	1.00%	0.25%	0.05%	1.30%
Institutional Shares	[1]	1.00%	none	0.04%	1.04%
R6 Shares	[2]	1.00%	none	0.04%	1.04%

Example

This example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example - BARON SMALL CAP FUND - USD ($)	1	3	5	10
Retail Shares	132	412	713	1,568
Institutional Shares	106	331	574	1,271
R6 Shares	106	331	574	1,271

Portfolio Turnover.

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes for Fund shareholders. These costs, which are not reflected in Total Annual Fund Operating Expenses or in the example, affect the Fund’s performance. During the most recent fiscal year ended September 30, 2015, the Fund’s portfolio turnover rate was 14.66% of the average value of its portfolio.

Investments, Risks, and Performance

Principal Investment Strategies of the Fund

The Fund invests 80% of its net assets in equity securities in the form of common stock of small-sized growth companies. BAMCO, Inc. (“BAMCO” or the “Adviser”) defines small-sized companies as those, at the time of purchase, with market capitalizations up to the largest market cap stock in the Russell 2000 Growth Index at reconstitution, or companies with market capitalizations up to $2.5 billion, whichever is larger. At the last reconstitution by Russell on June 30, 2015, the largest market cap stock in the Russell 2000 Growth Index was $4.71 billion. The Adviser seeks to invest in businesses it believes have significant opportunities for growth, sustainable competitive advantages, exceptional management, and an attractive valuation. Because of its long-term approach, the Fund could have a significant percentage of its assets invested in securities that have appreciated beyond their market capitalizations at the time of the Fund’s investment.

Principal Risks of Investing in the Fund

General Stock Market.  Investing in the stock market is risky because securities fluctuate in value. These fluctuations may be due to political, economic or general market circumstances. Other factors may affect a single company or industry but not the broader market. Because the values of securities fluctuate, when you sell your investment in the Fund, you may lose money. Current and future portfolio holdings in the Fund are subject to risk.

Small-Sized Companies.  The Adviser believes there is more potential for capital appreciation in small-sized companies, but there also may be more risk. Securities of small-sized companies may not be well known to most investors, and the securities may be less actively traded than those of large businesses. The securities of small-sized companies may fluctuate in price more widely than the stock market generally, and they may be more difficult to sell during market downturns. Small-sized companies rely more on the skills of management and on their continued tenure. Investing in small-sized companies requires a long-term outlook and may require shareholders to assume more risk and to have more patience than investing in the securities of larger, more established companies.

Special Situations.  The Funds may invest in “special situations.” A special situation arises when, in the opinion of the Adviser, the securities of a company will be recognized and appreciate in value due to a specific anticipated development at that company. Such developments might include a new product, a management change, an acquisition or a technological advancement. The risk of investing in special situations is that the anticipated development does not occur or its impact is not what the Adviser expected.

Growth Investing.  Growth stocks can react differently to issuer, political, market and economic developments than the market as a whole and other types of stocks. Growth stocks tend to be more expensive relative to their earnings or assets compared to other types of stocks. As a result, growth stocks tend to be sensitive to changes in their earnings and more volatile than other types of stocks.

Performance

The following bar chart and table provide some indication of the risks of investing in the Fund (Retail Shares) by showing changes in the Fund’s performance from year to year and by showing how the Fund’s average annual returns for 1, 5 and 10 years compare with those of a broad measure of market performance. The Fund’s past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. Updated performance information is available online at www.BaronFunds.com/performance or by calling 1-800-99BARON (1-800-992-2766).

Year by Year Total Return (%) as of December 31 of Each Year (Retail Shares)

Best Quarter:  06/30/09:  20.31%

Worst Quarter:  12/31/08: (23.85%)

Average Annual Total Returns (for periods ended 12/31/15)

The table below shows the Fund’s Retail Shares’ annual returns and long-term performance (before and after taxes) and the change in value of broad-based market indexes over various periods ended December 31, 2015. The table also shows the average annual returns of the Fund’s Institutional Shares, but it does not show after-tax returns.

After-tax returns are calculated using the highest individual federal marginal income tax rate in effect at the time of each distribution and assumed sale, but they do not include the impact of state and local taxes.

Your actual after-tax returns depend on your own tax situation and may differ from those shown. After-tax returns reflect past tax effects and are not predictive of future tax effects. After-tax returns are not relevant to investors who hold their Fund’s shares in a tax-deferred account (including a 401(k) or IRA or Coverdell account), or to investors that are tax-exempt.

Average Annual Total Returns for the periods ended December 31, 2015

Average Annual Total Returns - BARON SMALL CAP FUND		1 year	5 years	10 years	Since Inception	Inception date
Retail Shares		(5.24%)	9.04%	6.75%	9.19%	Sep. 30, 1997
Retail Shares | Return after taxes on distributions		(7.65%)	7.82%	5.85%	8.58%	Sep. 30, 1997
Retail Shares | Return after taxes on distributions and sale of Fund shares		(0.94%)	7.20%	5.50%	7.92%	Sep. 30, 1997
Institutional Shares	[1]	(5.01%)	9.31%	6.92%	9.29%	May 29, 2009
Russell 2000® Growth Index (reflects no deduction for fees, expenses or taxes)		(1.38%)	10.67%	7.95%	5.11%
S&P 500 Index (reflects no deduction for fees, expenses or taxes)		1.38%	12.57%	7.31%	6.26%

The Fund’s R6 Shares are newly issued and therefore have no performance as of the date of this prospectus. The performance of R6 Shares will differ from that shown above only to the extent that the classes do not have the same expenses.

The Russell 2000^® Growth Index is an unmanaged index of U.S. small-cap growth companies. The S&P 500 Index is an unmanaged index of larger-cap companies.

[1] Based on the fiscal year ended September 30, 2015.

[2] Because R6 Shares are newly issued, the Other Expenses are based on estimated amounts for the current fiscal year.

[1] Performance for the Institutional Shares prior to May 29, 2009 is based on the performance of the Retail Shares, which have a distribution fee. The Institutional Shares do not have a distribution fee. If the annual returns for the Institutional Shares prior to May 29, 2009 did not reflect this fee, the returns would be higher.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

BARON INVESTMENT FUNDS TRUST (f/k/a BARON ASSET FUND)

Prospectus Date

rr_ProspectusDate

Jan. 28, 2016

BARON SMALL CAP FUND

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Baron Small Cap Fund

Objective [Heading]

rr_ObjectiveHeading

Investment Goal

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The investment goal of Baron Small Cap Fund (the “Fund”) is capital appreciation through investments primarily in securities of small-sized growth companies.

Expense [Heading]

rr_ExpenseHeading

Fees and Expenses of the Fund

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The table below describes the fees and expenses that you would pay if you bought and held shares of the Fund.

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

Annual Fund Operating Expenses
(Expenses that you pay each year as a percentage of the value of your investment)

Portfolio Turnover [Heading]

rr_PortfolioTurnoverHeading

Portfolio Turnover.

Portfolio Turnover [Text Block]

rr_PortfolioTurnoverTextBlock

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes for Fund shareholders. These costs, which are not reflected in Total Annual Fund Operating Expenses or in the example, affect the Fund’s performance. During the most recent fiscal year ended September 30, 2015, the Fund’s portfolio turnover rate was 14.66% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

14.66%

Other Expenses, New Fund, Based on Estimates [Text]

rr_OtherExpensesNewFundBasedOnEstimates

Because R6 Shares are newly issued, the Other Expenses are based on estimated amounts for the current fiscal year.

Expense Example [Heading]

rr_ExpenseExampleHeading

Example

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Strategy [Heading]

rr_StrategyHeading

Investments, Risks, and Performance

Principal Investment Strategies of the Fund

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

The Fund invests 80% of its net assets in equity securities in the form of common stock of small-sized growth companies. BAMCO, Inc. (“BAMCO” or the “Adviser”) defines small-sized companies as those, at the time of purchase, with market capitalizations up to the largest market cap stock in the Russell 2000 Growth Index at reconstitution, or companies with market capitalizations up to $2.5 billion, whichever is larger. At the last reconstitution by Russell on June 30, 2015, the largest market cap stock in the Russell 2000 Growth Index was $4.71 billion. The Adviser seeks to invest in businesses it believes have significant opportunities for growth, sustainable competitive advantages, exceptional management, and an attractive valuation. Because of its long-term approach, the Fund could have a significant percentage of its assets invested in securities that have appreciated beyond their market capitalizations at the time of the Fund’s investment.

Risk [Heading]

rr_RiskHeading

Principal Risks of Investing in the Fund

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

General Stock Market.  Investing in the stock market is risky because securities fluctuate in value. These fluctuations may be due to political, economic or general market circumstances. Other factors may affect a single company or industry but not the broader market. Because the values of securities fluctuate, when you sell your investment in the Fund, you may lose money. Current and future portfolio holdings in the Fund are subject to risk.

Small-Sized Companies.  The Adviser believes there is more potential for capital appreciation in small-sized companies, but there also may be more risk. Securities of small-sized companies may not be well known to most investors, and the securities may be less actively traded than those of large businesses. The securities of small-sized companies may fluctuate in price more widely than the stock market generally, and they may be more difficult to sell during market downturns. Small-sized companies rely more on the skills of management and on their continued tenure. Investing in small-sized companies requires a long-term outlook and may require shareholders to assume more risk and to have more patience than investing in the securities of larger, more established companies.

Special Situations.  The Funds may invest in “special situations.” A special situation arises when, in the opinion of the Adviser, the securities of a company will be recognized and appreciate in value due to a specific anticipated development at that company. Such developments might include a new product, a management change, an acquisition or a technological advancement. The risk of investing in special situations is that the anticipated development does not occur or its impact is not what the Adviser expected.

Growth Investing.  Growth stocks can react differently to issuer, political, market and economic developments than the market as a whole and other types of stocks. Growth stocks tend to be more expensive relative to their earnings or assets compared to other types of stocks. As a result, growth stocks tend to be sensitive to changes in their earnings and more volatile than other types of stocks.

Risk Lose Money [Text]

rr_RiskLoseMoney

Because the values of securities fluctuate, when you sell your investment in the Fund, you may lose money.

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

Performance

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following bar chart and table provide some indication of the risks of investing in the Fund (Retail Shares) by showing changes in the Fund’s performance from year to year and by showing how the Fund’s average annual returns for 1, 5 and 10 years compare with those of a broad measure of market performance. The Fund’s past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. Updated performance information is available online at www.BaronFunds.com/performance or by calling 1-800-99BARON (1-800-992-2766).

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

The following bar chart and table provide some indication of the risks of investing in the Fund (Retail Shares) by showing changes in the Fund’s performance from year to year and by showing how the Fund’s average annual returns for 1, 5 and 10 years compare with those of a broad measure of market performance.

Performance One Year or Less [Text]

rr_PerformanceOneYearOrLess

The Fund’s R6 Shares are newly issued and therefore have no performance as of the date of this prospectus.

Performance Additional Market Index [Text]

rr_PerformanceAdditionalMarketIndex

The Russell 2000^® Growth Index is an unmanaged index of U.S. small-cap growth companies. The S&P 500 Index is an unmanaged index of larger-cap companies.

Performance Availability Phone [Text]

rr_PerformanceAvailabilityPhone

1-800-99BARON (1-800-992-2766)

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

www.BaronFunds.com/performance

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

The Fund’s past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future.

Bar Chart [Heading]

rr_BarChartHeading

Year by Year Total Return (%) as of December 31 of Each Year (Retail Shares)

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

Best Quarter:  06/30/09:  20.31%

Worst Quarter:  12/31/08: (23.85%)

Performance Table Heading

rr_PerformanceTableHeading

Average Annual Total Returns (for periods ended 12/31/15)

Performance Table Uses Highest Federal Rate

rr_PerformanceTableUsesHighestFederalRate

After-tax returns are calculated using the highest individual federal marginal income tax rate in effect at the time of each distribution and assumed sale, but they do not include the impact of state and local taxes.

Performance Table Not Relevant to Tax Deferred

rr_PerformanceTableNotRelevantToTaxDeferred

After-tax returns are not relevant to investors who hold their Fund’s shares in a tax-deferred account (including a 401(k) or IRA or Coverdell account), or to investors that are tax-exempt.

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

The table below shows the Fund’s Retail Shares’ annual returns and long-term performance (before and after taxes) and the change in value of broad-based market indexes over various periods ended December 31, 2015. The table also shows the average annual returns of the Fund’s Institutional Shares, but it does not show after-tax returns.

After-tax returns are calculated using the highest individual federal marginal income tax rate in effect at the time of each distribution and assumed sale, but they do not include the impact of state and local taxes.

Your actual after-tax returns depend on your own tax situation and may differ from those shown. After-tax returns reflect past tax effects and are not predictive of future tax effects. After-tax returns are not relevant to investors who hold their Fund’s shares in a tax-deferred account (including a 401(k) or IRA or Coverdell account), or to investors that are tax-exempt.

Performance Table Closing [Text Block]

rr_PerformanceTableClosingTextBlock

The Fund’s R6 Shares are newly issued and therefore have no performance as of the date of this prospectus. The performance of R6 Shares will differ from that shown above only to the extent that the classes do not have the same expenses.

The Russell 2000^® Growth Index is an unmanaged index of U.S. small-cap growth companies. The S&P 500 Index is an unmanaged index of larger-cap companies.

Caption

rr_AverageAnnualReturnCaption

Average Annual Total Returns for the periods ended December 31, 2015

BARON SMALL CAP FUND | Retail Shares

Risk/Return:

rr_RiskReturnAbstract

Management Fee

rr_ManagementFeesOverAssets

1.00%

^[1]

Distribution (12b-1) Fee

rr_DistributionAndService12b1FeesOverAssets

0.25%

^[1]

Other Expenses

rr_OtherExpensesOverAssets

0.05%

^[1]

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.30%

^[1]

1

rr_ExpenseExampleYear01

$ 132

3

rr_ExpenseExampleYear03

412

5

rr_ExpenseExampleYear05

713

10

rr_ExpenseExampleYear10

$ 1,568

2006

rr_AnnualReturn2006

11.83%

2007

rr_AnnualReturn2007

11.69%

2008

rr_AnnualReturn2008

(40.24%)

2009

rr_AnnualReturn2009

35.26%

2010

rr_AnnualReturn2010

23.47%

2011

rr_AnnualReturn2011

(1.58%)

2012

rr_AnnualReturn2012

17.98%

2013

rr_AnnualReturn2013

37.77%

2014

rr_AnnualReturn2014

1.69%

2015

rr_AnnualReturn2015

(5.24%)

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Best Quarter:

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Jun. 30, 2009

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

20.31%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Worst Quarter:

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Dec. 31, 2008

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(23.85%)

1 year

rr_AverageAnnualReturnYear01

(5.24%)

5 years

rr_AverageAnnualReturnYear05

9.04%

10 years

rr_AverageAnnualReturnYear10

6.75%

Since Inception

rr_AverageAnnualReturnSinceInception

9.19%

Inception date

rr_AverageAnnualReturnInceptionDate

Sep. 30, 1997

BARON SMALL CAP FUND | Institutional Shares

Risk/Return:

rr_RiskReturnAbstract

Management Fee

rr_ManagementFeesOverAssets

1.00%

^[1]

Distribution (12b-1) Fee

rr_DistributionAndService12b1FeesOverAssets

none

^[1]

Other Expenses

rr_OtherExpensesOverAssets

0.04%

^[1]

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.04%

^[1]

1

rr_ExpenseExampleYear01

$ 106

3

rr_ExpenseExampleYear03

331

5

rr_ExpenseExampleYear05

574

10

rr_ExpenseExampleYear10

$ 1,271

1 year

rr_AverageAnnualReturnYear01

(5.01%)

^[2]

5 years

rr_AverageAnnualReturnYear05

9.31%

^[2]

10 years

rr_AverageAnnualReturnYear10

6.92%

^[2]

Since Inception

rr_AverageAnnualReturnSinceInception

9.29%

^[2]

Inception date

rr_AverageAnnualReturnInceptionDate

May 29, 2009

^[2]

BARON SMALL CAP FUND | R6 Shares

Risk/Return:

rr_RiskReturnAbstract

Management Fee

rr_ManagementFeesOverAssets

1.00%

^[3]

Distribution (12b-1) Fee

rr_DistributionAndService12b1FeesOverAssets

none

^[3]

Other Expenses

rr_OtherExpensesOverAssets

0.04%

^[3]

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.04%

^[3]

1

rr_ExpenseExampleYear01

$ 106

3

rr_ExpenseExampleYear03

331

5

rr_ExpenseExampleYear05

574

10

rr_ExpenseExampleYear10

$ 1,271

BARON SMALL CAP FUND | Return after taxes on distributions | Retail Shares

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

(7.65%)

5 years

rr_AverageAnnualReturnYear05

7.82%

10 years

rr_AverageAnnualReturnYear10

5.85%

Since Inception

rr_AverageAnnualReturnSinceInception

8.58%

Inception date

rr_AverageAnnualReturnInceptionDate

Sep. 30, 1997

BARON SMALL CAP FUND | Return after taxes on distributions and sale of Fund shares | Retail Shares

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

(0.94%)

5 years

rr_AverageAnnualReturnYear05

7.20%

10 years

rr_AverageAnnualReturnYear10

5.50%

Since Inception

rr_AverageAnnualReturnSinceInception

7.92%

Inception date

rr_AverageAnnualReturnInceptionDate

Sep. 30, 1997

BARON SMALL CAP FUND | Russell 2000® Growth Index (reflects no deduction for fees, expenses or taxes)

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

(1.38%)

5 years

rr_AverageAnnualReturnYear05

10.67%

10 years

rr_AverageAnnualReturnYear10

7.95%

Since Inception

rr_AverageAnnualReturnSinceInception

5.11%

BARON SMALL CAP FUND | S&P 500 Index (reflects no deduction for fees, expenses or taxes)

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

1.38%

5 years

rr_AverageAnnualReturnYear05

12.57%

10 years

rr_AverageAnnualReturnYear10

7.31%

Since Inception

rr_AverageAnnualReturnSinceInception

6.26%

[1] Based on the fiscal year ended September 30, 2015.

[2] Performance for the Institutional Shares prior to May 29, 2009 is based on the performance of the Retail Shares, which have a distribution fee. The Institutional Shares do not have a distribution fee. If the annual returns for the Institutional Shares prior to May 29, 2009 did not reflect this fee, the returns would be higher.

[3] Because R6 Shares are newly issued, the Other Expenses are based on estimated amounts for the current fiscal year.

XML

BARON OPPORTUNITY FUND

Baron Opportunity Fund

Investment Goal

The investment goal of Baron Opportunity Fund (the “Fund”) is capital appreciation through investments primarily in growth companies that benefit from technology advances.

Fees and Expenses of the Fund

The table below describes the fees and expenses that you would pay if you bought and held shares of the Fund.

Annual Fund Operating Expenses
(Expenses that you pay each year as a percentage of the value of your investment)

Annual Fund Operating Expenses - BARON OPPORTUNITY FUND		Management Fee	Distribution (12b-1) Fee	Other Expenses	Total Annual Fund Operating Expenses
Retail Shares	[1]	1.00%	0.25%	0.13%	1.38%
Institutional Shares	[1]	1.00%	none	0.10%	1.10%

Example

This example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example - BARON OPPORTUNITY FUND - USD ($)	1	3	5	10
Retail Shares	140	437	755	1,657
Institutional Shares	112	350	606	1,340

Portfolio Turnover.

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes for Fund shareholders. These costs, which are not reflected in Total Annual Fund Operating Expenses or in the example, affect the Fund’s performance. During the most recent fiscal year ended September 30, 2015, the Fund’s portfolio turnover rate was 41.87% of the average value of its portfolio.

Investments, Risks, and Performance

Principal Investment Strategies of the Fund

The Fund invests primarily in equity securities in the form of common stock of high growth businesses of any market capitalization selected for their capital appreciation potential. The Adviser may invest in companies in any sector or industry that it believes will benefit from innovations and advances in technology. The Adviser seeks to invest in businesses it believes have significant opportunities for growth, sustainable competitive advantages, exceptional management, and an attractive valuation.

Principal Risks of Investing in the Fund

General Stock Market.  Investing in the stock market is risky because securities fluctuate in value. These fluctuations may be due to political, economic or general market circumstances. Other factors may affect a single company or industry but not the broader market. Because the values of securities fluctuate, when you sell your investment in the Fund, you may lose money. Current and future portfolio holdings in the Fund are subject to risk.

Technology.  Technology companies, including internet-related and information technology companies, as well as companies propelled by new technologies, may present the risk of rapid change and product obsolescence, and their successes may be difficult to predict for the long term. Some technology companies may be newly formed and have limited operating history and experience. Technology companies may also be adversely affected by changes in governmental policies, competitive pressures and changing demand. The securities of these companies may also experience significant price movements caused by disproportionate investor optimism or pessimism, with little or no basis in the companies' fundamentals or economic conditions.

Small- and Mid-Sized Companies.  The Adviser believes there is more potential for capital appreciation in small- and mid-sized companies, but there also may be more risk. Securities of small- and mid-sized companies may not be well known to most investors, and the securities may be less actively traded than those of large businesses. The securities of small- and mid-sized companies may fluctuate in price more widely than the stock market generally, and they may be more difficult to sell during market downturns. Small- and mid-sized companies rely more on the skills of management and on their continued tenure. Investing in small- and mid-sized companies requires a long-term outlook and may require shareholders to assume more risk and to have more patience than investing in the securities of larger, more established companies.

Growth Investing.  Growth stocks can react differently to issuer, political, market and economic developments than the market as a whole and other types of stocks. Growth stocks tend to be more expensive relative to their earnings or assets compared to other types of stocks. As a result, growth stocks tend to be sensitive to changes in their earnings and more volatile than other types of stocks.

Performance

The following bar chart and table provide some indication of the risks of investing in the Fund (Retail Shares) by showing changes in the Fund’s performance from year to year and by showing how the Fund’s average annual returns for 1, 5 and 10 years compare with those of a broad measure of market performance. The Fund’s past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. Updated performance information is available online at www.BaronFunds.com/performance or by calling 1-800-99BARON (1-800-992-2766).

Year by Year Total Return (%) as of December 31 of Each Year (Retail Shares)

Best Quarter:  06/30/09:  23.97%

Worst Quarter:  12/31/08: (28.08%)

Average Annual Total Returns (for periods ended 12/31/15)

The table below shows the Fund’s Retail Shares’ annual returns and long-term performance (before and after taxes) and the change in value of broad-based market indexes over various periods ended December 31, 2015. The table also shows the average annual returns of the Fund’s Institutional Shares, but it does not show after-tax returns.

After-tax returns are calculated using the highest individual federal marginal income tax rate in effect at the time of each distribution and assumed sale, but they do not include the impact of state and local taxes.

Your actual after-tax returns depend on your own tax situation and may differ from those shown. After-tax returns reflect past tax effects and are not predictive of future tax effects. After-tax returns are not relevant to investors who hold their Fund’s shares in a tax-deferred account (including a 401(k) or IRA or Coverdell account), or to investors that are tax-exempt.

Average Annual Total Returns for the periods ended December 31, 2015

Average Annual Total Returns - BARON OPPORTUNITY FUND		1 year	5 years	10 years	Since Inception	Inception date
Retail Shares		1.20%	7.87%	8.06%	4.85%	Feb. 29, 2000
Retail Shares | Return after taxes on distributions		(1.15%)	6.60%	7.42%	4.45%	Feb. 29, 2000
Retail Shares | Return after taxes on distributions and sale of Fund shares		2.65%	6.07%	6.52%	3.92%	Feb. 29, 2000
Institutional Shares	[1]	1.45%	8.15%	8.25%	4.97%	May 29, 2009
Russell 3000® Growth Index (reflects no deduction for fees, expenses or taxes)		5.09%	13.30%	8.49%	2.46%
S&P 500 Index (reflects no deduction for fees, expenses or taxes)		1.38%	12.57%	7.31%	4.57%
Russell Midcap® Growth Index (reflects no deduction for fees, expenses or taxes)	[2]	(0.20%)	11.54%	8.16%	3.45%

The Russell 3000^® Growth Index is an unmanaged index of companies classified as growth among the largest 3,000 U.S. companies. The S&P 500 Index is an unmanaged index of larger-cap companies. The Russell Midcap^® Growth Index is an unmanaged index of mid-cap growth companies.

[1] Based on the fiscal year ended September 30, 2015.

[1] Performance for the Institutional Shares prior to May 29, 2009 is based on the performance of the Retail Shares, which have a distribution fee. The Institutional Shares do not have a distribution fee. If the annual returns for the Institutional Shares prior to May 29, 2009 did not reflect this fee, the returns would be higher.

[2] Baron Opportunity Fund no longer considers the Russell Midcap^® Growth Index an appropriate benchmark index. The Russell Midcap^® Growth Index is included in the table above for comparison purposes for the period before Baron Opportunity Fund converted to an all-cap fund. Prior to February 20, 2015, the Fund invested in companies with market capitalizations between $1 billion and $15 billion at the time of purchase. Since then, the Fund may invest in companies of all market capitalizations. The Adviser believes that the Russell 3000^® Growth Index is more representative of the Fund's current investable universe.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

BARON INVESTMENT FUNDS TRUST (f/k/a BARON ASSET FUND)

Prospectus Date

rr_ProspectusDate

Jan. 28, 2016

BARON OPPORTUNITY FUND

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Baron Opportunity Fund

Objective [Heading]

rr_ObjectiveHeading

Investment Goal

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The investment goal of Baron Opportunity Fund (the “Fund”) is capital appreciation through investments primarily in growth companies that benefit from technology advances.

Expense [Heading]

rr_ExpenseHeading

Fees and Expenses of the Fund

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The table below describes the fees and expenses that you would pay if you bought and held shares of the Fund.

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

Annual Fund Operating Expenses
(Expenses that you pay each year as a percentage of the value of your investment)

Portfolio Turnover [Heading]

rr_PortfolioTurnoverHeading

Portfolio Turnover.

Portfolio Turnover [Text Block]

rr_PortfolioTurnoverTextBlock

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes for Fund shareholders. These costs, which are not reflected in Total Annual Fund Operating Expenses or in the example, affect the Fund’s performance. During the most recent fiscal year ended September 30, 2015, the Fund’s portfolio turnover rate was 41.87% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

41.87%

Expense Example [Heading]

rr_ExpenseExampleHeading

Example

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Strategy [Heading]

rr_StrategyHeading

Investments, Risks, and Performance

Principal Investment Strategies of the Fund

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

The Fund invests primarily in equity securities in the form of common stock of high growth businesses of any market capitalization selected for their capital appreciation potential. The Adviser may invest in companies in any sector or industry that it believes will benefit from innovations and advances in technology. The Adviser seeks to invest in businesses it believes have significant opportunities for growth, sustainable competitive advantages, exceptional management, and an attractive valuation.

Strategy Portfolio Concentration [Text]

rr_StrategyPortfolioConcentration

The Adviser may invest in companies in any sector or industry that it believes will benefit from innovations and advances in technology.

Risk [Heading]

rr_RiskHeading

Principal Risks of Investing in the Fund

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

General Stock Market.  Investing in the stock market is risky because securities fluctuate in value. These fluctuations may be due to political, economic or general market circumstances. Other factors may affect a single company or industry but not the broader market. Because the values of securities fluctuate, when you sell your investment in the Fund, you may lose money. Current and future portfolio holdings in the Fund are subject to risk.

Technology.  Technology companies, including internet-related and information technology companies, as well as companies propelled by new technologies, may present the risk of rapid change and product obsolescence, and their successes may be difficult to predict for the long term. Some technology companies may be newly formed and have limited operating history and experience. Technology companies may also be adversely affected by changes in governmental policies, competitive pressures and changing demand. The securities of these companies may also experience significant price movements caused by disproportionate investor optimism or pessimism, with little or no basis in the companies' fundamentals or economic conditions.

Small- and Mid-Sized Companies.  The Adviser believes there is more potential for capital appreciation in small- and mid-sized companies, but there also may be more risk. Securities of small- and mid-sized companies may not be well known to most investors, and the securities may be less actively traded than those of large businesses. The securities of small- and mid-sized companies may fluctuate in price more widely than the stock market generally, and they may be more difficult to sell during market downturns. Small- and mid-sized companies rely more on the skills of management and on their continued tenure. Investing in small- and mid-sized companies requires a long-term outlook and may require shareholders to assume more risk and to have more patience than investing in the securities of larger, more established companies.

Growth Investing.  Growth stocks can react differently to issuer, political, market and economic developments than the market as a whole and other types of stocks. Growth stocks tend to be more expensive relative to their earnings or assets compared to other types of stocks. As a result, growth stocks tend to be sensitive to changes in their earnings and more volatile than other types of stocks.

Risk Lose Money [Text]

rr_RiskLoseMoney

Because the values of securities fluctuate, when you sell your investment in the Fund, you may lose money.

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

Performance

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following bar chart and table provide some indication of the risks of investing in the Fund (Retail Shares) by showing changes in the Fund’s performance from year to year and by showing how the Fund’s average annual returns for 1, 5 and 10 years compare with those of a broad measure of market performance. The Fund’s past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. Updated performance information is available online at www.BaronFunds.com/performance or by calling 1-800-99BARON (1-800-992-2766).

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

The following bar chart and table provide some indication of the risks of investing in the Fund (Retail Shares) by showing changes in the Fund’s performance from year to year and by showing how the Fund’s average annual returns for 1, 5 and 10 years compare with those of a broad measure of market performance.

Performance Additional Market Index [Text]

rr_PerformanceAdditionalMarketIndex

The Russell 3000^® Growth Index is an unmanaged index of companies classified as growth among the largest 3,000 U.S. companies. The S&P 500 Index is an unmanaged index of larger-cap companies. The Russell Midcap^® Growth Index is an unmanaged index of mid-cap growth companies.

Performance Availability Phone [Text]

rr_PerformanceAvailabilityPhone

1-800-99BARON (1-800-992-2766)

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

www.BaronFunds.com/performance

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

The Fund’s past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future.

Bar Chart [Heading]

rr_BarChartHeading

Year by Year Total Return (%) as of December 31 of Each Year (Retail Shares)

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

Best Quarter:  06/30/09:  23.97%

Worst Quarter:  12/31/08: (28.08%)

Performance Table Heading

rr_PerformanceTableHeading

Average Annual Total Returns (for periods ended 12/31/15)

Performance Table Market Index Changed

rr_PerformanceTableMarketIndexChanged

Baron Opportunity Fund no longer considers the Russell Midcap^® Growth Index an appropriate benchmark index. The Russell Midcap^® Growth Index is included in the table above for comparison purposes for the period before Baron Opportunity Fund converted to an all-cap fund. Prior to February 20, 2015, the Fund invested in companies with market capitalizations between $1 billion and $15 billion at the time of purchase. Since then, the Fund may invest in companies of all market capitalizations. The Adviser believes that the Russell 3000^® Growth Index is more representative of the Fund's current investable universe.

Performance Table Uses Highest Federal Rate

rr_PerformanceTableUsesHighestFederalRate

After-tax returns are calculated using the highest individual federal marginal income tax rate in effect at the time of each distribution and assumed sale, but they do not include the impact of state and local taxes.

Performance Table Not Relevant to Tax Deferred

rr_PerformanceTableNotRelevantToTaxDeferred

After-tax returns are not relevant to investors who hold their Fund’s shares in a tax-deferred account (including a 401(k) or IRA or Coverdell account), or to investors that are tax-exempt.

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

The table below shows the Fund’s Retail Shares’ annual returns and long-term performance (before and after taxes) and the change in value of broad-based market indexes over various periods ended December 31, 2015. The table also shows the average annual returns of the Fund’s Institutional Shares, but it does not show after-tax returns.

After-tax returns are calculated using the highest individual federal marginal income tax rate in effect at the time of each distribution and assumed sale, but they do not include the impact of state and local taxes.

Your actual after-tax returns depend on your own tax situation and may differ from those shown. After-tax returns reflect past tax effects and are not predictive of future tax effects. After-tax returns are not relevant to investors who hold their Fund’s shares in a tax-deferred account (including a 401(k) or IRA or Coverdell account), or to investors that are tax-exempt.

Performance Table Closing [Text Block]

rr_PerformanceTableClosingTextBlock

The Russell 3000^® Growth Index is an unmanaged index of companies classified as growth among the largest 3,000 U.S. companies. The S&P 500 Index is an unmanaged index of larger-cap companies. The Russell Midcap^® Growth Index is an unmanaged index of mid-cap growth companies.

Caption

rr_AverageAnnualReturnCaption

Average Annual Total Returns for the periods ended December 31, 2015

BARON OPPORTUNITY FUND | Retail Shares

Risk/Return:

rr_RiskReturnAbstract

Management Fee

rr_ManagementFeesOverAssets

1.00%

^[1]

Distribution (12b-1) Fee

rr_DistributionAndService12b1FeesOverAssets

0.25%

^[1]

Other Expenses

rr_OtherExpensesOverAssets

0.13%

^[1]

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.38%

^[1]

1

rr_ExpenseExampleYear01

$ 140

3

rr_ExpenseExampleYear03

437

5

rr_ExpenseExampleYear05

755

10

rr_ExpenseExampleYear10

$ 1,657

2006

rr_AnnualReturn2006

12.14%

2007

rr_AnnualReturn2007

21.10%

2008

rr_AnnualReturn2008

(46.06%)

2009

rr_AnnualReturn2009

61.80%

2010

rr_AnnualReturn2010

25.43%

2011

rr_AnnualReturn2011

(6.16%)

2012

rr_AnnualReturn2012

13.69%

2013

rr_AnnualReturn2013

37.55%

2014

rr_AnnualReturn2014

(1.67%)

2015

rr_AnnualReturn2015

1.20%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Best Quarter:

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Jun. 30, 2009

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

23.97%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Worst Quarter:

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Dec. 31, 2008

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(28.08%)

1 year

rr_AverageAnnualReturnYear01

1.20%

5 years

rr_AverageAnnualReturnYear05

7.87%

10 years

rr_AverageAnnualReturnYear10

8.06%

Since Inception

rr_AverageAnnualReturnSinceInception

4.85%

Inception date

rr_AverageAnnualReturnInceptionDate

Feb. 29, 2000

BARON OPPORTUNITY FUND | Institutional Shares

Risk/Return:

rr_RiskReturnAbstract

Management Fee

rr_ManagementFeesOverAssets

1.00%

^[1]

Distribution (12b-1) Fee

rr_DistributionAndService12b1FeesOverAssets

none

^[1]

Other Expenses

rr_OtherExpensesOverAssets

0.10%

^[1]

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.10%

^[1]

1

rr_ExpenseExampleYear01

$ 112

3

rr_ExpenseExampleYear03

350

5

rr_ExpenseExampleYear05

606

10

rr_ExpenseExampleYear10

$ 1,340

1 year

rr_AverageAnnualReturnYear01

1.45%

^[2]

5 years

rr_AverageAnnualReturnYear05

8.15%

^[2]

10 years

rr_AverageAnnualReturnYear10

8.25%

^[2]

Since Inception

rr_AverageAnnualReturnSinceInception

4.97%

^[2]

Inception date

rr_AverageAnnualReturnInceptionDate

May 29, 2009

^[2]

BARON OPPORTUNITY FUND | Return after taxes on distributions | Retail Shares

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

(1.15%)

5 years

rr_AverageAnnualReturnYear05

6.60%

10 years

rr_AverageAnnualReturnYear10

7.42%

Since Inception

rr_AverageAnnualReturnSinceInception

4.45%

Inception date

rr_AverageAnnualReturnInceptionDate

Feb. 29, 2000

BARON OPPORTUNITY FUND | Return after taxes on distributions and sale of Fund shares | Retail Shares

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

2.65%

5 years

rr_AverageAnnualReturnYear05

6.07%

10 years

rr_AverageAnnualReturnYear10

6.52%

Since Inception

rr_AverageAnnualReturnSinceInception

3.92%

Inception date

rr_AverageAnnualReturnInceptionDate

Feb. 29, 2000

BARON OPPORTUNITY FUND | Russell 3000® Growth Index (reflects no deduction for fees, expenses or taxes)

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

5.09%

5 years

rr_AverageAnnualReturnYear05

13.30%

10 years

rr_AverageAnnualReturnYear10

8.49%

Since Inception

rr_AverageAnnualReturnSinceInception

2.46%

BARON OPPORTUNITY FUND | S&P 500 Index (reflects no deduction for fees, expenses or taxes)

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

1.38%

5 years

rr_AverageAnnualReturnYear05

12.57%

10 years

rr_AverageAnnualReturnYear10

7.31%

Since Inception

rr_AverageAnnualReturnSinceInception

4.57%

BARON OPPORTUNITY FUND | Russell Midcap® Growth Index (reflects no deduction for fees, expenses or taxes)

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

(0.20%)

^[3]

5 years

rr_AverageAnnualReturnYear05

11.54%

^[3]

10 years

rr_AverageAnnualReturnYear10

8.16%

^[3]

Since Inception

rr_AverageAnnualReturnSinceInception

3.45%

^[3]

[1] Based on the fiscal year ended September 30, 2015.

[2] Performance for the Institutional Shares prior to May 29, 2009 is based on the performance of the Retail Shares, which have a distribution fee. The Institutional Shares do not have a distribution fee. If the annual returns for the Institutional Shares prior to May 29, 2009 did not reflect this fee, the returns would be higher.

[3] Baron Opportunity Fund no longer considers the Russell Midcap^® Growth Index an appropriate benchmark index. The Russell Midcap^® Growth Index is included in the table above for comparison purposes for the period before Baron Opportunity Fund converted to an all-cap fund. Prior to February 20, 2015, the Fund invested in companies with market capitalizations between $1 billion and $15 billion at the time of purchase. Since then, the Fund may invest in companies of all market capitalizations. The Adviser believes that the Russell 3000^® Growth Index is more representative of the Fund's current investable universe.

XML

BARON FIFTH AVENUE GROWTH FUND

Baron Fifth Avenue Growth Fund

Investment Goal

The investment goal of Baron Fifth Avenue Growth Fund (the “Fund”) is capital appreciation through investments primarily in securities of large-sized growth companies.

Fees and Expenses of the Fund

The table below describes the fees and expenses that you would pay if you bought and held shares of the Fund.

Annual Fund Operating Expenses
(Expenses that you pay each year as a percentage of the value of your investment)

Annual Fund Operating Expenses - BARON FIFTH AVENUE GROWTH FUND		Management Fee	Distribution (12b-1) Fee	Other Expenses	Total Annual Fund Operating Expenses	Expense Waivers	Total Annual Fund Operating Expenses After Expense Waivers	[2]
Retail Shares	[1]	0.90%	0.25%	0.17%	1.32%	(0.02%)	1.30%
Institutional Shares	[1]	0.90%	none	0.14%	1.04%		1.04%
R6 Shares	[3]	0.90%	none	0.12%	1.02%		1.02%

Example

This example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same at 1.30% for the Retail Shares, 1.04% for the Institutional Shares and 1.02% for the R6 Shares. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example - BARON FIFTH AVENUE GROWTH FUND - USD ($)	1	3	5	10
Retail Shares	132	412	713	1,568
Institutional Shares	106	331	574	1,271
R6 Shares	104	325	563	1,248

Portfolio Turnover.

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes for Fund shareholders. These costs, which are not reflected in Total Annual Fund Operating Expenses or in the example, affect the Fund’s performance. During the most recent fiscal year ended September 30, 2015, the Fund’s portfolio turnover rate was 14.54% of the average value of its portfolio.

Investments, Risks, and Performance

Principal Investment Strategies of the Fund

The Fund invests primarily in equity securities in the form of common stock of large-sized growth companies. The Adviser defines large-sized companies as those, at the time of purchase, with market capitalizations above the smallest market cap stock in the top 85% of the Russell 1000 Growth Index at reconstitution, or companies with market capitalizations above $10 billion, whichever is smaller. At the last reconstitution by Russell on June 30, 2015, the smallest market cap stock in the top 85% of the Russell 1000 Growth Index was $11.88 billion. The Adviser seeks to invest in businesses it believes have significant opportunities for growth, sustainable competitive advantages, exceptional management, and an attractive valuation.

Principal Risks of Investing in the Fund

General Stock Market.  Investing in the stock market is risky because securities fluctuate in value. These fluctuations may be due to political, economic or general market circumstances. Other factors may affect a single company or industry but not the broader market. Because the values of securities fluctuate, when you sell your investment in the Fund, you may lose money. Current and future portfolio holdings in the Fund are subject to risk.

Growth Investing.  Growth stocks can react differently to issuer, political, market and economic developments than the market as a whole and other types of stocks. Growth stocks tend to be more expensive relative to their earnings or assets compared to other types of stocks. As a result, growth stocks tend to be sensitive to changes in their earnings and more volatile than other types of stocks.

Performance

The following bar chart and table provide some indication of the risks of investing in the Fund (Retail Shares) by showing changes in the Fund’s performance from year to year and by showing how the Fund’s average annual returns for 1, 5 and 10 years compare with those of a broad measure of market performance. The Fund’s past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. Updated performance information is available online at www.BaronFunds.com/performance or by calling 1-800-99BARON (1-800-992-2766).

Year by Year Total Return (%) as of December 31 of Each Year (Retail Shares)

Best Quarter:  03/31/12:  22.69%

Worst Quarter:  12/31/08: (25.16%)

Average Annual Total Returns (for periods ended 12/31/15)

The table below shows the Fund’s Retail Shares’ annual returns and long-term performance (before and after taxes) and the change in value of broad-based market indexes over various periods ended December 31, 2015. The table also shows the average annual returns of the Fund’s Institutional Shares, but it does not show after-tax returns.

After-tax returns are calculated using the highest individual federal marginal income tax rate in effect at the time of each distribution and assumed sale, but they do not include the impact of state and local taxes.

Your actual after-tax returns depend on your own tax situation and may differ from those shown. After-tax returns reflect past tax effects and are not predictive of future tax effects. After-tax returns are not relevant to investors who hold their Fund’s shares in a tax-deferred account (including a 401(k) or IRA or Coverdell account), or to investors that are tax-exempt.

Average Annual Total Returns for the periods ended December 31, 2015

Average Annual Total Returns - BARON FIFTH AVENUE GROWTH FUND		1 year	5 years	10 years	Since Inception	Inception date
Retail Shares		6.33%	12.88%	6.57%	7.19%	Apr. 30, 2004
Retail Shares | Return after taxes on distributions		6.33%	12.88%	6.26%	6.93%	Apr. 30, 2004
Retail Shares | Return after taxes on distributions and sale of Fund shares		3.58%	10.32%	5.27%	5.86%	Apr. 30, 2004
Institutional Shares	[1]	6.61%	13.17%	6.74%	7.34%	May 29, 2009
Russell 1000® Growth Index (reflects no deduction for fees, expenses or taxes)		5.67%	13.53%	8.53%	8.35%
S&P 500 Index (reflects no deduction for fees, expenses or taxes)		1.38%	12.57%	7.31%	7.61%

The Fund’s R6 Shares are newly issued and therefore have no performance as of the date of this prospectus. The performance of R6 Shares will differ from that shown above only to the extent that the classes do not have the same expenses.

The Russell 1000^® Growth Index is an unmanaged index of larger-cap growth companies. The S&P 500 Index is an unmanaged index of larger-cap companies.

[1] Based on the fiscal year ended September 30, 2015.

[2] BAMCO, Inc. ("BAMCO" or the "Adviser") has agreed that for so long as it serves as the adviser to the Fund, it will reimburse certain expenses of the Fund, limiting net annual operating expenses (expenses such as portfolio transaction costs, interest, dividend and extraordinary expenses are not subject to the operating expense limitation) to 1.30% of average daily net assets of Retail Shares, 1.05% of average daily net assets of Institutional Shares and 1.04% of average daily net assets of R6 shares.

[3] Because R6 Shares are newly issued, the Other Expenses are based on estimated amounts for the current fiscal year.

[1] Performance for the Institutional Shares prior to May 29, 2009 is based on the performance of the Retail Shares, which have a distribution fee. The Institutional Shares do not have a distribution fee. If the annual returns for the Institutional Shares prior to May 29, 2009 did not reflect this fee, the returns would be higher.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

BARON INVESTMENT FUNDS TRUST (f/k/a BARON ASSET FUND)

Prospectus Date

rr_ProspectusDate

Jan. 28, 2016

BARON FIFTH AVENUE GROWTH FUND

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Baron Fifth Avenue Growth Fund

Objective [Heading]

rr_ObjectiveHeading

Investment Goal

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The investment goal of Baron Fifth Avenue Growth Fund (the “Fund”) is capital appreciation through investments primarily in securities of large-sized growth companies.

Expense [Heading]

rr_ExpenseHeading

Fees and Expenses of the Fund

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The table below describes the fees and expenses that you would pay if you bought and held shares of the Fund.

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

Annual Fund Operating Expenses
(Expenses that you pay each year as a percentage of the value of your investment)

Portfolio Turnover [Heading]

rr_PortfolioTurnoverHeading

Portfolio Turnover.

Portfolio Turnover [Text Block]

rr_PortfolioTurnoverTextBlock

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes for Fund shareholders. These costs, which are not reflected in Total Annual Fund Operating Expenses or in the example, affect the Fund’s performance. During the most recent fiscal year ended September 30, 2015, the Fund’s portfolio turnover rate was 14.54% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

14.54%

Other Expenses, New Fund, Based on Estimates [Text]

rr_OtherExpensesNewFundBasedOnEstimates

Because R6 Shares are newly issued, the Other Expenses are based on estimated amounts for the current fiscal year.

Expense Example [Heading]

rr_ExpenseExampleHeading

Example

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same at 1.30% for the Retail Shares, 1.04% for the Institutional Shares and 1.02% for the R6 Shares. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Strategy [Heading]

rr_StrategyHeading

Investments, Risks, and Performance

Principal Investment Strategies of the Fund

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

The Fund invests primarily in equity securities in the form of common stock of large-sized growth companies. The Adviser defines large-sized companies as those, at the time of purchase, with market capitalizations above the smallest market cap stock in the top 85% of the Russell 1000 Growth Index at reconstitution, or companies with market capitalizations above $10 billion, whichever is smaller. At the last reconstitution by Russell on June 30, 2015, the smallest market cap stock in the top 85% of the Russell 1000 Growth Index was $11.88 billion. The Adviser seeks to invest in businesses it believes have significant opportunities for growth, sustainable competitive advantages, exceptional management, and an attractive valuation.

Risk [Heading]

rr_RiskHeading

Principal Risks of Investing in the Fund

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

General Stock Market.  Investing in the stock market is risky because securities fluctuate in value. These fluctuations may be due to political, economic or general market circumstances. Other factors may affect a single company or industry but not the broader market. Because the values of securities fluctuate, when you sell your investment in the Fund, you may lose money. Current and future portfolio holdings in the Fund are subject to risk.

Growth Investing.  Growth stocks can react differently to issuer, political, market and economic developments than the market as a whole and other types of stocks. Growth stocks tend to be more expensive relative to their earnings or assets compared to other types of stocks. As a result, growth stocks tend to be sensitive to changes in their earnings and more volatile than other types of stocks.

Risk Lose Money [Text]

rr_RiskLoseMoney

Because the values of securities fluctuate, when you sell your investment in the Fund, you may lose money.

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

Performance

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following bar chart and table provide some indication of the risks of investing in the Fund (Retail Shares) by showing changes in the Fund’s performance from year to year and by showing how the Fund’s average annual returns for 1, 5 and 10 years compare with those of a broad measure of market performance. The Fund’s past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. Updated performance information is available online at www.BaronFunds.com/performance or by calling 1-800-99BARON (1-800-992-2766).

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

The following bar chart and table provide some indication of the risks of investing in the Fund (Retail Shares) by showing changes in the Fund’s performance from year to year and by showing how the Fund’s average annual returns for 1, 5 and 10 years compare with those of a broad measure of market performance.

Performance One Year or Less [Text]

rr_PerformanceOneYearOrLess

The Fund’s R6 Shares are newly issued and therefore have no performance as of the date of this prospectus.

Performance Additional Market Index [Text]

rr_PerformanceAdditionalMarketIndex

The Russell 1000^® Growth Index is an unmanaged index of larger-cap growth companies. The S&P 500 Index is an unmanaged index of larger-cap companies.

Performance Availability Phone [Text]

rr_PerformanceAvailabilityPhone

1-800-99BARON (1-800-992-2766)

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

www.BaronFunds.com/performance

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

The Fund’s past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future.

Bar Chart [Heading]

rr_BarChartHeading

Year by Year Total Return (%) as of December 31 of Each Year (Retail Shares)

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

Best Quarter:  03/31/12:  22.69%

Worst Quarter:  12/31/08: (25.16%)

Performance Table Heading

rr_PerformanceTableHeading

Average Annual Total Returns (for periods ended 12/31/15)

Performance Table Uses Highest Federal Rate

rr_PerformanceTableUsesHighestFederalRate

After-tax returns are calculated using the highest individual federal marginal income tax rate in effect at the time of each distribution and assumed sale, but they do not include the impact of state and local taxes.

Performance Table Not Relevant to Tax Deferred

rr_PerformanceTableNotRelevantToTaxDeferred

After-tax returns are not relevant to investors who hold their Fund’s shares in a tax-deferred account (including a 401(k) or IRA or Coverdell account), or to investors that are tax-exempt.

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

The table below shows the Fund’s Retail Shares’ annual returns and long-term performance (before and after taxes) and the change in value of broad-based market indexes over various periods ended December 31, 2015. The table also shows the average annual returns of the Fund’s Institutional Shares, but it does not show after-tax returns.

After-tax returns are calculated using the highest individual federal marginal income tax rate in effect at the time of each distribution and assumed sale, but they do not include the impact of state and local taxes.

Your actual after-tax returns depend on your own tax situation and may differ from those shown. After-tax returns reflect past tax effects and are not predictive of future tax effects. After-tax returns are not relevant to investors who hold their Fund’s shares in a tax-deferred account (including a 401(k) or IRA or Coverdell account), or to investors that are tax-exempt.

Performance Table Closing [Text Block]

rr_PerformanceTableClosingTextBlock

The Fund’s R6 Shares are newly issued and therefore have no performance as of the date of this prospectus. The performance of R6 Shares will differ from that shown above only to the extent that the classes do not have the same expenses.

The Russell 1000^® Growth Index is an unmanaged index of larger-cap growth companies. The S&P 500 Index is an unmanaged index of larger-cap companies.

Caption

rr_AverageAnnualReturnCaption

Average Annual Total Returns for the periods ended December 31, 2015

BARON FIFTH AVENUE GROWTH FUND | Retail Shares

Risk/Return:

rr_RiskReturnAbstract

Management Fee

rr_ManagementFeesOverAssets

0.90%

^[1]

Distribution (12b-1) Fee

rr_DistributionAndService12b1FeesOverAssets

0.25%

^[1]

Other Expenses

rr_OtherExpensesOverAssets

0.17%

^[1]

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.32%

^[1]

Expense Waivers

rr_FeeWaiverOrReimbursementOverAssets

(0.02%)

^[1]

Total Annual Fund Operating Expenses After Expense Waivers

rr_NetExpensesOverAssets

1.30%

^[1],[2]

1

rr_ExpenseExampleYear01

$ 132

3

rr_ExpenseExampleYear03

412

5

rr_ExpenseExampleYear05

713

10

rr_ExpenseExampleYear10

$ 1,568

2006

rr_AnnualReturn2006

10.17%

2007

rr_AnnualReturn2007

5.64%

2008

rr_AnnualReturn2008

(38.92%)

2009

rr_AnnualReturn2009

31.15%

2010

rr_AnnualReturn2010

10.57%

2011

rr_AnnualReturn2011

(2.58%)

2012

rr_AnnualReturn2012

20.85%

2013

rr_AnnualReturn2013

35.44%

2014

rr_AnnualReturn2014

8.08%

2015

rr_AnnualReturn2015

6.33%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Best Quarter:

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Mar. 31, 2012

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

22.69%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Worst Quarter:

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Dec. 31, 2008

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(25.16%)

1 year

rr_AverageAnnualReturnYear01

6.33%

5 years

rr_AverageAnnualReturnYear05

12.88%

10 years

rr_AverageAnnualReturnYear10

6.57%

Since Inception

rr_AverageAnnualReturnSinceInception

7.19%

Inception date

rr_AverageAnnualReturnInceptionDate

Apr. 30, 2004

BARON FIFTH AVENUE GROWTH FUND | Institutional Shares

Risk/Return:

rr_RiskReturnAbstract

Management Fee

rr_ManagementFeesOverAssets

0.90%

^[1]

Distribution (12b-1) Fee

rr_DistributionAndService12b1FeesOverAssets

none

^[1]

Other Expenses

rr_OtherExpensesOverAssets

0.14%

^[1]

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.04%

^[1]

Expense Waivers

rr_FeeWaiverOrReimbursementOverAssets

^[1]

Total Annual Fund Operating Expenses After Expense Waivers

rr_NetExpensesOverAssets

1.04%

^[1],[2]

1

rr_ExpenseExampleYear01

$ 106

3

rr_ExpenseExampleYear03

331

5

rr_ExpenseExampleYear05

574

10

rr_ExpenseExampleYear10

$ 1,271

1 year

rr_AverageAnnualReturnYear01

6.61%

^[3]

5 years

rr_AverageAnnualReturnYear05

13.17%

^[3]

10 years

rr_AverageAnnualReturnYear10

6.74%

^[3]

Since Inception

rr_AverageAnnualReturnSinceInception

7.34%

^[3]

Inception date

rr_AverageAnnualReturnInceptionDate

May 29, 2009

^[3]

BARON FIFTH AVENUE GROWTH FUND | R6 Shares

Risk/Return:

rr_RiskReturnAbstract

Management Fee

rr_ManagementFeesOverAssets

0.90%

^[4]

Distribution (12b-1) Fee

rr_DistributionAndService12b1FeesOverAssets

none

^[4]

Other Expenses

rr_OtherExpensesOverAssets

0.12%

^[4]

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.02%

^[4]

Expense Waivers

rr_FeeWaiverOrReimbursementOverAssets

^[4]

Total Annual Fund Operating Expenses After Expense Waivers

rr_NetExpensesOverAssets

1.02%

^[2],[4]

1

rr_ExpenseExampleYear01

$ 104

3

rr_ExpenseExampleYear03

325

5

rr_ExpenseExampleYear05

563

10

rr_ExpenseExampleYear10

$ 1,248

BARON FIFTH AVENUE GROWTH FUND | Return after taxes on distributions | Retail Shares

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

6.33%

5 years

rr_AverageAnnualReturnYear05

12.88%

10 years

rr_AverageAnnualReturnYear10

6.26%

Since Inception

rr_AverageAnnualReturnSinceInception

6.93%

Inception date

rr_AverageAnnualReturnInceptionDate

Apr. 30, 2004

BARON FIFTH AVENUE GROWTH FUND | Return after taxes on distributions and sale of Fund shares | Retail Shares

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

3.58%

5 years

rr_AverageAnnualReturnYear05

10.32%

10 years

rr_AverageAnnualReturnYear10

5.27%

Since Inception

rr_AverageAnnualReturnSinceInception

5.86%

Inception date

rr_AverageAnnualReturnInceptionDate

Apr. 30, 2004

BARON FIFTH AVENUE GROWTH FUND | Russell 1000® Growth Index (reflects no deduction for fees, expenses or taxes)

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

5.67%

5 years

rr_AverageAnnualReturnYear05

13.53%

10 years

rr_AverageAnnualReturnYear10

8.53%

Since Inception

rr_AverageAnnualReturnSinceInception

8.35%

BARON FIFTH AVENUE GROWTH FUND | S&P 500 Index (reflects no deduction for fees, expenses or taxes)

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

1.38%

5 years

rr_AverageAnnualReturnYear05

12.57%

10 years

rr_AverageAnnualReturnYear10

7.31%

Since Inception

rr_AverageAnnualReturnSinceInception

7.61%

[1] Based on the fiscal year ended September 30, 2015.

[2] BAMCO, Inc. ("BAMCO" or the "Adviser") has agreed that for so long as it serves as the adviser to the Fund, it will reimburse certain expenses of the Fund, limiting net annual operating expenses (expenses such as portfolio transaction costs, interest, dividend and extraordinary expenses are not subject to the operating expense limitation) to 1.30% of average daily net assets of Retail Shares, 1.05% of average daily net assets of Institutional Shares and 1.04% of average daily net assets of R6 shares.

[3] Performance for the Institutional Shares prior to May 29, 2009 is based on the performance of the Retail Shares, which have a distribution fee. The Institutional Shares do not have a distribution fee. If the annual returns for the Institutional Shares prior to May 29, 2009 did not reflect this fee, the returns would be higher.

[4] Because R6 Shares are newly issued, the Other Expenses are based on estimated amounts for the current fiscal year.

XML

BARON DISCOVERY FUND

Baron Discovery Fund

Investment Goal

The investment goal of Baron Discovery Fund (the “Fund”) is capital appreciation through investments primarily in securities of small-sized growth companies.

Fees and Expenses of the Fund

The table below describes the fees and expenses that you would pay if you bought and held shares of the Fund.

Annual Fund Operating Expenses
(Expenses that you pay each year as a percentage of the value of your investment)

Annual Fund Operating Expenses - BARON DISCOVERY FUND		Management Fee	Distribution (12b-1) Fee	Other Expenses	Total Annual Fund Operating Expenses	Expense Waivers	Total Annual Fund Operating Expenses After Expense Waivers	[2]
Retail Shares	[1]	1.00%	0.25%	0.32%	1.57%	(0.22%)	1.35%
Institutional Shares	[1]	1.00%	none	0.25%	1.25%	(0.15%)	1.10%

Example

This example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same at 1.35% for the Retail Shares and 1.10% for the Institutional Shares. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example - BARON DISCOVERY FUND - USD ($)	1	3	5	10
Retail Shares	137	428	739	1,624
Institutional Shares	112	350	606	1,340

Portfolio Turnover.

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes for Fund shareholders. These costs, which are not reflected in Total Annual Fund Operating Expenses or in the example, affect the Fund’s performance. During the most recent fiscal year ended September 30, 2015, the Fund’s portfolio turnover rate was 114.82% of the average value of its portfolio.

Investments, Risks, and Performance

Principal Investment Strategies of the Fund

The Fund invests primarily in equity securities in the form of common stock of small-sized growth companies. The Adviser defines small-sized companies as those, at the time of purchase, with market capitalizations up to the weighted median market capitalization of the Russell 2000 Growth Index at reconstitution, or companies with market capitalizations up to $1.5 billion, whichever is larger. At the last reconstitution by Russell on June 30, 2015, the weighted median market cap of the Russell 2000 Growth Index was $2.01 billion. The Adviser seeks to invest in businesses it believes have significant opportunities for growth, sustainable competitive advantages, exceptional management, and an attractive valuation.

Principal Risks of Investing in the Fund

General Stock Market.  Investing in the stock market is risky because securities fluctuate in value. These fluctuations may be due to political, economic or general market circumstances. Other factors may affect a single company or industry but not the broader market. Because the values of securities fluctuate, when you sell your investment in the Fund, you may lose money. Current and future portfolio holdings in the Fund are subject to risk.

Small-Sized Companies.  The Adviser believes there is more potential for capital appreciation in small-sized companies, but there also may be more risk. Securities of small-sized companies may not be well known to most investors, and the securities may be less actively traded than those of large businesses. The securities of small-sized companies may fluctuate in price more widely than the stock market generally, and they may be more difficult to sell during market downturns. Small-sized companies rely more on the skills of management and on their continued tenure. Investing in small-sized companies requires a long-term outlook and may require shareholders to assume more risk and to have more patience than investing in the securities of larger, more established companies.

Special Situations.  The Funds may invest in "special situations." A special situation arises when, in the opinion of the Adviser, the securities of a company will be recognized and appreciate in value due to a specific anticipated development at that company. Such developments might include a new product, a management change, an acquisition or a technological advancement. The risk of investing in special situations is that the anticipated development does not occur or its impact is not what the Adviser expected.

Growth Investing.  Growth stocks can react differently to issuer, political, market and economic developments than the market as a whole and other types of stocks. Growth stocks tend to be more expensive relative to their earnings or assets compared to other types of stocks. As a result, growth stocks tend to be sensitive to changes in their earnings and more volatile than other types of stocks.

Performance

The following bar chart and table provide some indication of the risks of investing in the Fund (Retail Shares) by showing changes in the Fund’s performance from year to year and by showing how the Fund’s average annual returns compare with those of a broad measure of market performance. The Fund’s past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. Updated performance information is available online at www.BaronFunds.com/performance or by calling 1-800-99BARON (1-800-992-2766).

Year by Year Total Return (%) as of December 31 of Each Year (Retail Shares)

Best Quarter:  12/31/14: 13.36%

Worst Quarter:  9/30/15:  (19.87%)

Average Annual Total Returns (for periods ended 12/31/15)

The table below shows the Fund's Retail Shares' annual returns and long-term performance (before and after taxes) and the change in value of broad-based market indexes over various periods ended December 31, 2015. The table also shows the average annual returns of the Fund's Institutional Shares, but it does not show after-tax returns.

After-tax returns are calculated using the highest individual federal marginal income tax rate in effect at the time of each distribution and assumed sale, but they do not include the impact of state and local taxes.

Your actual after-tax returns depend on your own tax situation and may differ from those shown. After-tax returns reflect past tax effects and are not predictive of future tax effects. After-tax returns are not relevant to investors who hold their Fund's shares in a tax-deferred account (including a 401(k) or IRA or Coverdell account), or to investors that are tax-exempt.

Average Annual Total Returns for the periods ended December 31, 2015

Average Annual Total Returns - BARON DISCOVERY FUND	1 year	5 years	10 years	Since Inception	Inception date
Retail Shares	(14.73%)			5.54%	Sep. 30, 2013
Retail Shares | Return after taxes on distributions	(14.73%)			5.54%	Sep. 30, 2013
Retail Shares | Return after taxes on distributions and sale of Fund shares	(8.34%)			4.26%	Sep. 30, 2013
Institutional Shares	(14.53%)			5.79%	Sep. 30, 2013
Russell 2000® Growth Index (reflects no deduction for fees, expenses or taxes)	(1.38%)			5.44%
S&P 500 Index (reflects no deduction for fees, expenses or taxes)	1.38%			11.36%

The Russell 2000^® Growth Index is an unmanaged index of U.S. small-cap growth companies. The S&P 500 Index is an unmanaged index of larger-cap companies.

[1] Based on the fiscal year ended September 30, 2015.

[2] BAMCO, Inc. ("BAMCO" or the "Adviser") has agreed that for so long as it serves as the adviser to the Fund, it will reimburse certain expenses of the Fund, limiting net annual operating expenses (expenses such as portfolio transaction costs, interest, dividend and extraordinary expenses are not subject to the operating expense limitation) to 1.35% of average daily net assets of Retail Shares and 1.10% of average daily net assets of Institutional Shares.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

BARON INVESTMENT FUNDS TRUST (f/k/a BARON ASSET FUND)

Prospectus Date

rr_ProspectusDate

Jan. 28, 2016

BARON DISCOVERY FUND

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Baron Discovery Fund

Objective [Heading]

rr_ObjectiveHeading

Investment Goal

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The investment goal of Baron Discovery Fund (the “Fund”) is capital appreciation through investments primarily in securities of small-sized growth companies.

Expense [Heading]

rr_ExpenseHeading

Fees and Expenses of the Fund

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The table below describes the fees and expenses that you would pay if you bought and held shares of the Fund.

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

Annual Fund Operating Expenses
(Expenses that you pay each year as a percentage of the value of your investment)

Portfolio Turnover [Heading]

rr_PortfolioTurnoverHeading

Portfolio Turnover.

Portfolio Turnover [Text Block]

rr_PortfolioTurnoverTextBlock

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes for Fund shareholders. These costs, which are not reflected in Total Annual Fund Operating Expenses or in the example, affect the Fund’s performance. During the most recent fiscal year ended September 30, 2015, the Fund’s portfolio turnover rate was 114.82% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

114.82%

Expense Example [Heading]

rr_ExpenseExampleHeading

Example

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same at 1.35% for the Retail Shares and 1.10% for the Institutional Shares. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Strategy [Heading]

rr_StrategyHeading

Investments, Risks, and Performance

Principal Investment Strategies of the Fund

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

The Fund invests primarily in equity securities in the form of common stock of small-sized growth companies. The Adviser defines small-sized companies as those, at the time of purchase, with market capitalizations up to the weighted median market capitalization of the Russell 2000 Growth Index at reconstitution, or companies with market capitalizations up to $1.5 billion, whichever is larger. At the last reconstitution by Russell on June 30, 2015, the weighted median market cap of the Russell 2000 Growth Index was $2.01 billion. The Adviser seeks to invest in businesses it believes have significant opportunities for growth, sustainable competitive advantages, exceptional management, and an attractive valuation.

Risk [Heading]

rr_RiskHeading

Principal Risks of Investing in the Fund

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

General Stock Market.  Investing in the stock market is risky because securities fluctuate in value. These fluctuations may be due to political, economic or general market circumstances. Other factors may affect a single company or industry but not the broader market. Because the values of securities fluctuate, when you sell your investment in the Fund, you may lose money. Current and future portfolio holdings in the Fund are subject to risk.

Small-Sized Companies.  The Adviser believes there is more potential for capital appreciation in small-sized companies, but there also may be more risk. Securities of small-sized companies may not be well known to most investors, and the securities may be less actively traded than those of large businesses. The securities of small-sized companies may fluctuate in price more widely than the stock market generally, and they may be more difficult to sell during market downturns. Small-sized companies rely more on the skills of management and on their continued tenure. Investing in small-sized companies requires a long-term outlook and may require shareholders to assume more risk and to have more patience than investing in the securities of larger, more established companies.

Special Situations.  The Funds may invest in "special situations." A special situation arises when, in the opinion of the Adviser, the securities of a company will be recognized and appreciate in value due to a specific anticipated development at that company. Such developments might include a new product, a management change, an acquisition or a technological advancement. The risk of investing in special situations is that the anticipated development does not occur or its impact is not what the Adviser expected.

Growth Investing.  Growth stocks can react differently to issuer, political, market and economic developments than the market as a whole and other types of stocks. Growth stocks tend to be more expensive relative to their earnings or assets compared to other types of stocks. As a result, growth stocks tend to be sensitive to changes in their earnings and more volatile than other types of stocks.

Risk Lose Money [Text]

rr_RiskLoseMoney

Because the values of securities fluctuate, when you sell your investment in the Fund, you may lose money.

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

Performance

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following bar chart and table provide some indication of the risks of investing in the Fund (Retail Shares) by showing changes in the Fund’s performance from year to year and by showing how the Fund’s average annual returns compare with those of a broad measure of market performance. The Fund’s past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. Updated performance information is available online at www.BaronFunds.com/performance or by calling 1-800-99BARON (1-800-992-2766).

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

The following bar chart and table provide some indication of the risks of investing in the Fund (Retail Shares) by showing changes in the Fund’s performance from year to year and by showing how the Fund’s average annual returns compare with those of a broad measure of market performance.

Performance Additional Market Index [Text]

rr_PerformanceAdditionalMarketIndex

The Russell 2000^® Growth Index is an unmanaged index of U.S. small-cap growth companies. The S&P 500 Index is an unmanaged index of larger-cap companies.

Performance Availability Phone [Text]

rr_PerformanceAvailabilityPhone

1-800-99BARON (1-800-992-2766)

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

www.BaronFunds.com/performance

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

The Fund’s past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future.

Bar Chart [Heading]

rr_BarChartHeading

Year by Year Total Return (%) as of December 31 of Each Year (Retail Shares)

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

Best Quarter:  12/31/14: 13.36%

Worst Quarter:  9/30/15:  (19.87%)

Performance Table Heading

rr_PerformanceTableHeading

Average Annual Total Returns (for periods ended 12/31/15)

Performance Table Uses Highest Federal Rate

rr_PerformanceTableUsesHighestFederalRate

After-tax returns are calculated using the highest individual federal marginal income tax rate in effect at the time of each distribution and assumed sale, but they do not include the impact of state and local taxes.

Performance Table Not Relevant to Tax Deferred

rr_PerformanceTableNotRelevantToTaxDeferred

After-tax returns are not relevant to investors who hold their Fund's shares in a tax-deferred account (including a 401(k) or IRA or Coverdell account), or to investors that are tax-exempt.

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

The table below shows the Fund's Retail Shares' annual returns and long-term performance (before and after taxes) and the change in value of broad-based market indexes over various periods ended December 31, 2015. The table also shows the average annual returns of the Fund's Institutional Shares, but it does not show after-tax returns.

After-tax returns are calculated using the highest individual federal marginal income tax rate in effect at the time of each distribution and assumed sale, but they do not include the impact of state and local taxes.

Your actual after-tax returns depend on your own tax situation and may differ from those shown. After-tax returns reflect past tax effects and are not predictive of future tax effects. After-tax returns are not relevant to investors who hold their Fund's shares in a tax-deferred account (including a 401(k) or IRA or Coverdell account), or to investors that are tax-exempt.

Performance Table Closing [Text Block]

rr_PerformanceTableClosingTextBlock

The Russell 2000^® Growth Index is an unmanaged index of U.S. small-cap growth companies. The S&P 500 Index is an unmanaged index of larger-cap companies.

Caption

rr_AverageAnnualReturnCaption

Average Annual Total Returns for the periods ended December 31, 2015

BARON DISCOVERY FUND | Retail Shares

Risk/Return:

rr_RiskReturnAbstract

Management Fee

rr_ManagementFeesOverAssets

1.00%

^[1]

Distribution (12b-1) Fee

rr_DistributionAndService12b1FeesOverAssets

0.25%

^[1]

Other Expenses

rr_OtherExpensesOverAssets

0.32%

^[1]

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.57%

^[1]

Expense Waivers

rr_FeeWaiverOrReimbursementOverAssets

(0.22%)

^[1]

Total Annual Fund Operating Expenses After Expense Waivers

rr_NetExpensesOverAssets

1.35%

^[1],[2]

1

rr_ExpenseExampleYear01

$ 137

3

rr_ExpenseExampleYear03

428

5

rr_ExpenseExampleYear05

739

10

rr_ExpenseExampleYear10

$ 1,624

2014

rr_AnnualReturn2014

13.45%

2015

rr_AnnualReturn2015

(14.73%)

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Best Quarter:

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2014

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

13.36%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Worst Quarter:

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Sep. 30, 2015

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(19.87%)

1 year

rr_AverageAnnualReturnYear01

(14.73%)

5 years

rr_AverageAnnualReturnYear05

10 years

rr_AverageAnnualReturnYear10

Since Inception

rr_AverageAnnualReturnSinceInception

5.54%

Inception date

rr_AverageAnnualReturnInceptionDate

Sep. 30, 2013

BARON DISCOVERY FUND | Institutional Shares

Risk/Return:

rr_RiskReturnAbstract

Management Fee

rr_ManagementFeesOverAssets

1.00%

^[1]

Distribution (12b-1) Fee

rr_DistributionAndService12b1FeesOverAssets

none

^[1]

Other Expenses

rr_OtherExpensesOverAssets

0.25%

^[1]

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.25%

^[1]

Expense Waivers

rr_FeeWaiverOrReimbursementOverAssets

(0.15%)

^[1]

Total Annual Fund Operating Expenses After Expense Waivers

rr_NetExpensesOverAssets

1.10%

^[1],[2]

1

rr_ExpenseExampleYear01

$ 112

3

rr_ExpenseExampleYear03

350

5

rr_ExpenseExampleYear05

606

10

rr_ExpenseExampleYear10

$ 1,340

1 year

rr_AverageAnnualReturnYear01

(14.53%)

5 years

rr_AverageAnnualReturnYear05

10 years

rr_AverageAnnualReturnYear10

Since Inception

rr_AverageAnnualReturnSinceInception

5.79%

Inception date

rr_AverageAnnualReturnInceptionDate

Sep. 30, 2013

BARON DISCOVERY FUND | Return after taxes on distributions | Retail Shares

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

(14.73%)

5 years

rr_AverageAnnualReturnYear05

10 years

rr_AverageAnnualReturnYear10

Since Inception

rr_AverageAnnualReturnSinceInception

5.54%

Inception date

rr_AverageAnnualReturnInceptionDate

Sep. 30, 2013

BARON DISCOVERY FUND | Return after taxes on distributions and sale of Fund shares | Retail Shares

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

(8.34%)

5 years

rr_AverageAnnualReturnYear05

10 years

rr_AverageAnnualReturnYear10

Since Inception

rr_AverageAnnualReturnSinceInception

4.26%

Inception date

rr_AverageAnnualReturnInceptionDate

Sep. 30, 2013

BARON DISCOVERY FUND | Russell 2000® Growth Index (reflects no deduction for fees, expenses or taxes)

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

(1.38%)

5 years

rr_AverageAnnualReturnYear05

10 years

rr_AverageAnnualReturnYear10

Since Inception

rr_AverageAnnualReturnSinceInception

5.44%

BARON DISCOVERY FUND | S&P 500 Index (reflects no deduction for fees, expenses or taxes)

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

1.38%

5 years

rr_AverageAnnualReturnYear05

10 years

rr_AverageAnnualReturnYear10

Since Inception

rr_AverageAnnualReturnSinceInception

11.36%

[1] Based on the fiscal year ended September 30, 2015.

[2] BAMCO, Inc. ("BAMCO" or the "Adviser") has agreed that for so long as it serves as the adviser to the Fund, it will reimburse certain expenses of the Fund, limiting net annual operating expenses (expenses such as portfolio transaction costs, interest, dividend and extraordinary expenses are not subject to the operating expense limitation) to 1.35% of average daily net assets of Retail Shares and 1.10% of average daily net assets of Institutional Shares.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	BARON INVESTMENT FUNDS TRUST (f/k/a BARON ASSET FUND)
Prospectus Date	rr_ProspectusDate	Jan. 28, 2016
Document Creation Date	dei_DocumentCreationDate	Jan. 28, 2016