The Company’s 2016 Equity Incentive Plan (the “2016 Plan”) became effective on September 15, 2016. The 2016 Plan provides for the grant of incentive stock options, non-qualified stock options, restricted stock awards, restricted stock unit awards, stock appreciation rights and performance share awards to employees, directors and consultants of the Company. The number of shares of common stock reserved for issuance under the 2016 Plan will automatically increase on January 1 of each year by 3% of the number of shares of the Company’s capital stock outstanding on the immediately preceding December 31, or such lesser number of shares as determined by the Company’s board of directors.

The Company’s Employee Stock Purchase Plan (the “2016 ESPP”) became effective on September 15, 2016. The number of shares reserved for issuance under the 2016 ESPP will automatically increase on January 1 of each year by the lesser of 200,000 shares of the Company’s common stock, 1% of the number of shares of the Company’s common stock outstanding on the immediately preceding December 31, or such lesser number of shares as determined by the Company’s board of directors.

The 2016 ESPP allows eligible employees to purchase shares of the Company’s common stock at a discount of up to 15% through payroll deductions of their eligible compensation, subject to any plan limitations. The 2016 ESPP provides for separate six-month offering periods beginning each March and September of each fiscal year.

On each purchase date, eligible employees will purchase the Company’s stock at a price per share equal to 85% of the lesser of (i) the fair market value of the Company’s common stock on the offering date or (ii) the fair market value of the Company’s common stock on the purchase date.

For the six months ended June 30, 2020 and 2019, 30,943 and 24,266 shares of common stock were purchased under the 2016 ESPP, respectively. The Company recorded stock-based compensation expense of $0.3 million and $0.2 million for the three months ended June 30, 2020 and 2019, respectively. The Company recorded stock-based compensation expense of $0.6 million and $0.5 million for the six months ended June 30, 2020 and 2019, respectively. As of June 30, 2020, unrecognized compensation cost related to the 2016 ESPP was $0.2 million which will be amortized over a weighted-average period of 0.21 years.

Stock option awards are granted with an exercise price equal to the fair market value of the Company’s common stock at the date of grant based on the closing market price of its common stock as reported on The Nasdaq Global Market. The option awards generally vest over four years and are exercisable any time after vesting. The stock options expire ten years after the date of grant.

There were no stock options granted during the six months ended June 30, 2020 and 2019. The Company recorded stock-based compensation expense of $1.0 million and $1.6 million for the three months ended June 30, 2020 and 2019, respectively, attributed to stock options. The Company recorded stock-based compensation expense of $1.9 million and $3.5 million for the six months ended June 30, 2020 and 2019, respectively, attributed to stock options.

The total intrinsic value of options exercised for the six months ended June 30, 2020 and 2019 was $15.4 million and $37.1 million, respectively. This intrinsic value represents the difference between the fair market value of the Company’s common stock on the date of exercise and the exercise price of each option. Based on the fair market value of the Company’s common stock at June 30, 2020 and 2019, the total intrinsic value of all outstanding options was $59.4 million and $61.7 million, respectively.

The fair value of shares issuable under the ESPP is determined using the Black-Scholes option pricing model with the following weighted average assumptions:

	Six Months Ended June 30,
	2020		2019
Employee Stock Purchase Plan:
Expected term (in years) ⁽¹⁾		0.50		0.50
Expected volatility ⁽²⁾	55%		45%
Risk-free interest rate ⁽³⁾	0.29%		2.52%
Dividend rate ⁽⁴⁾	0%		0%

Total unrecognized compensation cost related to nonvested stock options was approximately $4.8 million as of June 30, 2020 and is expected to be recognized over a weighted average period of 1.4 years. The amount of cash received from the exercise of stock options during the six months ended June 30, 2020 and 2019 was $4.6 million and $13.5 million, respectively.

	Stock options outstanding			Weighted average exercise price
Outstanding at December 31, 2019		723,383		$	26.56
Exercised		(186,215	)		24.63
Forfeited		(2,993	)		32.23
Outstanding at June 30, 2020		534,175			27.21

Stock options outstanding, vested and expected to vest and exercisable are as follows:

	As of June 30, 2020
	Number of shares		Remaining contractual life (years)		Weighted- average exercise price
Outstanding		534,175		7.01	$	27.21
Vested and expected to vest		525,042		7.00		27.11
Exercisable		180,975		6.16		20.09

Vested

Nonvested

Options

outstanding

Weighted

average

exercise

price

Options

outstanding

Weighted

average

exercise

price

Outstanding at June 30, 2020

180,975

20.09

353,200

30.85

During the six months ended June 30, 2020, the Company granted 127,037 restricted stock units (“RSUs”) to members of its senior management and certain other employees pursuant to the 2016 Plan. The Company accounts for RSUs issued to employees at fair value, based on the market price of the Company’s common stock on the date of grant. The weighted-average grant date fair values of RSUs granted during the six months ended June 30, 2020 and 2019 were $109.31 and $64.12, respectively. The fair values of RSUs that vested during the six months ended June 30, 2020 and 2019, were $4.0 million and $3.6 million, respectively. During the three months ended June 30, 2020 and 2019, the Company recorded $4.8 million and $4.6 million, respectively, of stock-based compensation related to the RSUs. During the six months ended June 30, 2020 and 2019, the Company recorded $9.3 million and $8.4 million, respectively, of stock-based compensation related to the RSUs. There were 66,113 RSUs that vested during the six months ended June 30, 2020.

As of June 30, 2020, there was $37.9 million of unrecognized compensation expense related to unvested RSUs which is expected to be recognized over a weighted-average period of approximately 2.47 years. For RSUs subject to graded vesting, the Company recognizes compensation cost on a straight-line basis over the service period for the entire award.

During the six months ended June 30, 2020, the Company granted 118,422 Performance-based restricted stock units (“PSUs”) to members of its management pursuant to the 2016 Plan. The PSUs generally vest based on the Company achieving certain revenue growth thresholds which range from 20% to 40% compounded annual growth over a measurement period of two years for the first 50% of PSUs and three years for the remaining PSUs. The vesting of the PSUs is subject to the employee’s continued employment with the Company through the date of achievement. During the six months ended June 30, 2020, the share price of the Company’s common stock on the date of issuance of the PSUs ranged from $82.23 to $158.95 per share. During the six months ended June 30, 2019, the share price of the Company’s common stock on the date of issuance of the PSUs ranged from $54.83 to $85.63 per share. The fair value is based on the value of the Company’s common stock at the date of issuance and the probability of achieving the performance metric. The Company has assessed the probability of achievement of the award at 100% based on past performance of achievement of the performance metric. Compensation cost is adjusted in future periods for subsequent changes in the expected outcome of the performance related conditions. The weighted-average grant date fair values of PSUs granted during the six months ended June 30, 2020 and 2019 were $108.57 and $62.03, respectively. No PSUs vested during the six months ended June 30, 2020 and 2019. During the three months ended June 30, 2020 and 2019, the Company recognized $5.2 million and $1.8 million, respectively, of stock compensation expense in connection with the PSU awards. During the six months ended June 30, 2020 and 2019, the Company recognized $9.8 million and $3.7 million, respectively, of stock compensation expense in connection with the PSU awards.

As of June 30, 2020, there was $31.6 million of unrecognized compensation expense related to unvested PSUs which is expected to be recognized over a weighted-average period of approximately 1.82 years. Compensation cost is recognized under the accelerated method and is adjusted in future periods for subsequent changes in the expected outcome of the performance related conditions.

The following table summarizes the Company’s RSU and PSU activity for the six months ended June 30, 2020:

	Number of Shares
Outstanding at December 31, 2019		1,314,791
Granted		245,459
Vested		(66,113	)
Forfeited		(9,424	)
Outstanding at June 30, 2020		1,484,713

The Company recorded the total stock-based compensation expense as follows (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2020		2019		2020		2019
Cost of revenue	$	703	$	412	$	1,311	$	847
Sales and marketing		3,917		2,547		7,525		4,915
Research and development		2,298		2,692		4,172		4,166
General and administrative		4,360		2,631		8,580		6,203
Total	$	11,278	$	8,282	$	21,588	$	16,131

Stock-based compensation expense is recognized over the award’s expected vesting schedule, which is reduced for forfeitures.