The Company sponsors the 2018 Equity Incentive Plan (the “2018 Plan”), which was approved by stockholders on September 5, 2018. The purpose of the 2018 Plan is to promote the long-term growth and profitability of the Company by (i) providing employees with incentives to improve stockholder value and to contribute to the growth and financial success of the Company through their future services, and (ii) enabling the Company to attract, retain and reward the best‑available persons. The options granted under the 2018 Plan, may be granted at a price not less than the fair market value on the grant date.

The Board, or a committee of the Board, has granted options with an exercise price at or which approximates the fair value on the grant date. Grants of time-based awards generally vest over a four-year period for new hires and over a three-year period for subsequent grants to existing employees. Options expire as determined by the Board, or committee of the Board, but not more than ten years after the date of the grant.

The 2018 Plan provides for annual increases in the number of shares available for issuance on the first day of each year equal to the lesser of (i) 12,500,000 shares, (ii) 5% of the outstanding shares on the last date of the preceding year, and (iii) a lower amount determined by the plan administrator. As of June 30, 2020, 15,004,326 shares of common stock remain available for grant under the 2018 Plan.

The following is a summary of restricted stock units for the current year period:

	Restricted Stock Units
	Number of Shares			Weighted Average Grant-Date Fair Value		Weighted Average Remaining Contractual Term (in years)
Unvested at December 31, 2019		6,975,994		$	14.72		2.2
Granted		3,999,194		$	18.21
Vested		(1,995,721	)	$	14.32
Forfeited/cancelled		(438,509	)	$	15.58
Unvested at June 30, 2020		8,540,958		$	16.40		2.4

The following is a summary of stock option activity for the current year period:

	Stock Options
	Number of Shares			Weighted Average Exercise Price		Aggregate Intrinsic Value (in thousands)		Weighted Average Remaining Contractual Term (in years)
Outstanding at December 31, 2019		15,812,928		$	14.67	$	50,994		7.4
Granted		2,312,230		$	21.15
Exercised		(1,920,482	)	$	13.34
Forfeited		(112,088	)	$	13.43
Expired		(23,376	)	$	14.73
Outstanding, vested and expected to vest at June 30, 2020		16,069,212		$	15.77	$	124,927		7.4
Vested and exercisable at June 30, 2020		9,760,821		$	15.15	$	81,850		6.4

The following is a summary of restricted stock awards for the current year period:

	Restricted Stock Awards
	Number of Shares			Weighted Average Grant-Date Fair Value		Weighted Average Remaining Contractual Term (in years)
Unvested at December 31, 2019		299,798		$	18.30		2.3
Vested		(99,933	)
Unvested at June 30, 2020		199,865		$	18.30		1.8

The Company used the Black-Scholes-Merton option pricing model to estimate the fair value of stock options granted using the following weighted-average assumptions:

	Three Months Ended June 30,		Six Months Ended June 30,
	2020	2019	2020	2019
Expected life (in years)	6.2	6.0	5.8	5.9
Risk-free interest rate	0.5%	2.2%	1.4%	2.5%
Volatility	52%	46%	48%	46%
Dividend yield	—%	—%	—%	—%
Fair value of common stock	$19.24	$16.48	$21.15	$12.73

The Company sponsors the 2018 Employee Stock Purchase Plan, as amended (the “ESPP”), which was first approved by stockholders on September 5, 2018. The ESPP provides for annual increases in the number of shares available for issuance on the first day of each year equal to the lesser of (i) 5,346,888 shares, (ii) 1% of the outstanding shares on the last date of the preceding year, and (iii) a lower amount determined by the plan administrator.

Except for the initial offering period, the ESPP provides for 24-month offering periods beginning May 22 and November 22 of each year, and each offering period will consist of four six-month purchase periods. The initial offering period began on September 25, 2018 and will end on November 22, 2020. On each purchase date, eligible employees will purchase the shares at a price per share equal to 85% of the lesser of (1) the fair market value of the Company’s common stock on the offering date, or (2) the fair market value of its common stock on the purchase date. During the three and six months ended June 30, 2020, the Company’s employees purchased 267,757 shares of its common stock under the ESPP at a weighted average purchase price of $11.51 with proceeds of $3.1 million. During the three and six months ended June 30, 2019, the Company’s employees purchased 260,991 shares of its common stock under the ESPP with a weighted average purchase price of $10.20 with proceeds of $2.7 million.

As of June 30, 2020, 4,518,860 shares of common stock remain available for issuance under the ESPP.

The Company used Black-Scholes-Merton option pricing model to estimate the fair value of ESPP purchase rights granted using the following weighted-average assumptions:

	Three Months Ended June 30,		Six Months Ended June 30,
	2020	2019	2020	2019
Expected life (in years)	1.3	1.2	1.3	1.2
Risk-free interest rate	0.2%	2.3%	0.2%	2.3%
Volatility	58%	43%	58%	43%
Dividend yield	—%	—%	—%	—%
Fair value of common stock	$19.81	$16.98	$19.81	$16.98

Stock-based compensation expense recognized in the condensed consolidated financial statements is as follows:

	Three Months Ended June 30,				Six Months Ended June 30,
(in thousands)	2020		2019		2020		2019
Cost of revenue	$	1,047	$	991	$	2,007	$	2,087
Research and development		7,496		5,629		13,953		10,395
Sales and marketing		4,841		3,016		9,184		5,796
General and administrative		6,087		5,518		11,829		11,987
Stock-based compensation expense, net of amounts capitalized		19,471		15,154		36,973		30,265
Capitalized stock-based compensation expense		710		1,078		1,486		2,031
Stock-based compensation expense	$	20,181	$	16,232	$	38,459	$	32,296

	Unrecognized stock-based compensation (in thousands)		Weighted average vesting period (in years)
Restricted stock units (service-based)	$	116,445		2.7
Restricted stock units (performance-based)⁽¹⁾		2,672		1.0
Stock options		44,198		2.4
Restricted stock awards		3,204		1.8
ESPP		3,667		1.3
Total unrecognized stock-based compensation	$	170,186

In the United States, the Company offers its employees a defined contribution plan that qualifies as a deferred salary arrangement under Section 401 of the U.S. Internal Revenue Code (“401(k) Plan”). Under the 401(k) Plan, participating employees may defer a portion of their pretax earnings not to exceed the maximum amount allowed by the Internal Revenue Service. The Company currently provides a matching contribution of 25% of deferrals for eligible employees. Compensation expense for the Company's matching contributions was $1.0 million and $2.2 million during the three and six months ended June 30, 2020, respectively, and $0.6 million and $1.6 million during the three and six months ended June 30, 2019, respectively.