We recorded stock-based compensation expense related to grants to employees, including those in connection with modified awards, on our condensed consolidated statements of operations as follows (in thousands):


	Three Months Ended June 30,				Six Months Ended June 30,
	2022		2021		2022		2021
Cost of revenue	$	11,877	$	5,340	$	20,671	$	10,457
Research and development	60,110		33,227		115,363		64,877
Sales and marketing	25,125		14,523		48,959		26,560
General and administrative	21,130		32,310		36,676		50,067
Total stock-based compensation expense	$	118,242	$	85,400	$	221,669	$	151,961

A summary of our stock option activity under the 2009 Stock Plan, 2019 Stock Plan, and 2020 Equity Incentive Plan (the "2020 Plan") is as follows:


	Options Outstanding
	Stock Options Outstanding	Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Term (In Years)
Balance as of December 31, 2021	29,226,041	$	13.28	6.26
Granted	1,569,938	$	74.59
Exercised	(3,038,757)	$	6.22
Forfeited, cancelled, or expired	(570,719)	$	34.77
Balance as of June 30, 2022	27,186,503	$	17.16	6.02

The calculated grant-date fair value of stock options granted was estimated using the Black-Scholes option-pricing model with the following assumptions:


	Three Months Ended June 30,		Six Months Ended June 30,
	2022	2021	2022	2021
Expected dividend yield	—	—	—	—
Risk-free interest rate	3.0%	1.1% - 1.2%	1.7% - 3.0%	0.9% - 1.2%
Expected volatility	33.3%	36.1% - 36.2%	33.3% - 33.8%	36.0% - 36.2%
Expected term (in years)	6.25	6.25	6.25	6.25
Fair value of underlying common stock	$38.47	$100.60 - $103.86	$38.47 - $89.01	$100.60 - $108.10

A summary of our RSU activity under the 2019 Stock Plan and 2020 Plan is as follows:


	Unvested RSUs
	Number of Shares	Weighted-Average Grant-Date Fair Value
Unvested as of December 31, 2021	13,696,836	$	85.96
Granted	6,089,462	$	66.22
Vested	(2,052,726)	$	83.49
Forfeited	(881,543)	$	84.69
Unvested as of June 30, 2022	16,852,029	$	79.20

In determining the fair value of the shares subject to our Employee Stock Purchase Plan (the "ESPP"), we use the Black-Scholes option pricing model that employs the following weighted-average assumptions:


									Six Months Ended June 30,
									2022
Expected dividend yield									—
Risk-free interest rate									0.6%
Expected volatility									40.0%
Expected term (in years)									0.50
Estimated fair value									$27.42

Additional information related to the ESPP is provided below (in thousands, except per share amounts):


			Six Months Ended June 30,
			2022
Share issued under the ESPP			207,986
Weighted-average price per share issued			$90.48

No shares were granted or issued under the ESPP during the three months ended June 30, 2022 or June 30, 2021.