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 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.

As of March 31, 2021, 17,094,581 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, 2020		7,000,855		$	17.72		1.1
Granted		2,764,398		$	26.89
Vested		(966,637	)	$	16.29
Forfeited/cancelled		(469,916	)	$	18.65
Unvested at March 31, 2021		8,328,700		$	20.88		1.2

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, 2020		15,100,142		$	16.07	$	143,156		6.9
Granted		2,472,878		$	21.99
Exercised		(641,252	)	$	14.93
Forfeited		(641,986	)	$	19.50
Expired		(12,117	)	$	13.91
Outstanding, vested and expected to vest at March 31, 2021		16,277,665		$	16.88	$	39,588		7.0
Vested and exercisable at March 31, 2021		10,607,672		$	15.46	$	32,604		5.9

On February 16, 2021, the Company granted 580,911 options to purchase shares of the Company’s common stock to its Chief Executive Officer at an exercise price of $21.99 per share (the “Executive Option Grant”). The stock options contain a service condition and a market condition of achieving a per share price milestone. The grant-date fair value of the award is $5.7 million, which will be recognized using the accelerated attribution method. Stock-based compensation is recognized for each vesting tranche using service periods that range from 9 months to 3 years. The grant-date fair value was determined using the Monte Carlo valuation, which incorporates various assumptions including expected stock price volatility, contractual term, dividend yield, and stock price at grant date. During the three months ended March 31, 2021, the Company recognized $0.5 million of stock-based compensation expense related to this award, which is included in general and administrative expense in the condensed consolidated statement of operations.

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, 2020		102,460		$	18.30		1.2
Granted		274,417		$	26.89
Vested		(17,077	)	$	18.30
Forfeited/cancelled		(64,823	)	$	26.89
Unvested at March 31, 2021		294,977		$	24.40		2.3

Except for the Executive Option Grant discussed above, 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 March 31,
	2021	2020
Expected life (in years)	5.8	5.8
Risk-free interest rate	0.7%	1.4%
Volatility	52%	48%
Dividend yield	—%	—%
Fair value of common stock	$21.99	$21.32

The Company sponsors the 2018 Employee Stock Purchase Plan, as amended (the “ESPP”), which was approved by stockholders on September 5, 2018. 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. 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.

As of March 31, 2021, 5,661,918 shares of common stock remain available for issuance under the ESPP.

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

	Three Months Ended March 31,
(in thousands)	2021		2020
Cost of revenue	$	1,482	$	960
Research and development		9,497		6,457
Sales and marketing		5,778		4,343
General and administrative		6,842		5,742
Stock-based compensation expense, net of amounts capitalized		23,599		17,502
Capitalized stock-based compensation expense		559		776
Stock-based compensation expense	$	24,158	$	18,278

	Unrecognized stock-based compensation (in thousands)		Weighted average vesting period (in years)
Restricted stock units	$	158,993		2.5
Stock options		48,201		2.4
Restricted stock awards		6,659		2.5
ESPP		5,084		1.5
Total unrecognized stock-based compensation	$	218,937

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 $1.4 million and $1.2 million during the three months ended March 31, 2021 and 2020, respectively.