The following table summarizes the components of stock-based compensation expense in the condensed consolidated statements of operations (in thousands):


	Three Months Ended March 31,
	2022		2021

Cost of revenue	$	3,860	$	2,999
Research and development	7,082		4,908
Sales and marketing	4,775		4,085
General and administrative	10,591		5,218
	$	26,308	$	17,210

The following table summarizes the stock option activity under our stock plans during the reporting period:


	Outstanding Options
	Number of Shares	Weighted Average Exercise Price		Remaining Contractual Life (Years)	Aggregate Intrinsic Value

					(in thousands)












Balances at December 31, 2021	10,737,295	$	21.23	5.2	$	60,304

Exercised	(149,662)
Cancelled	(188,289)
Balances at March 31, 2022	10,399,344	21.31		5.0	67,413
Vested and expected to vest at March 31, 2022	10,322,713	21.40		5.0	66,271
Exercisable at March 31, 2022	8,841,572	23.30		4.7	45,009

Stock Options - During the three months ended March 31, 2022 and 2021, we recognized $2.1 million and $4.0 million of stock-based compensation expense for stock options, respectively. We did not grant options in the three months ended March 31, 2022.

As of March 31, 2022 and 2021, we had unrecognized compensation expense related to unvested stock options of $4.4 million and $16.8 million, respectively. This expense is expected to be recognized over the remaining weighted-average period of 0.6 years and 1.6 years, respectively. Cash received from stock options exercised totaled $1.0 million and $53.2 million for the three months ended March 31, 2022 and 2021, respectively.


	Number of Awards Outstanding	Weighted Average Grant Date Fair Value













Unvested Balance at December 31, 2021	8,367,664	$	20.52
Granted	3,841,604	19.82
Vested	(964,937)	15.95
Forfeited	(89,251)	18.53
Unvested Balance at March 31, 2022	11,155,080	20.07

Stock Awards - The estimated fair value of restricted stock units ("RSUs") and performance stock units ("PSUs") is based on the fair value of our Class A common stock on the date of grant. For the three months ended March 31, 2022 and 2021, we recognized $21.0 million and $10.7 million of stock-based compensation expense for stock awards, respectively.

As of March 31, 2022 and 2021, we had $192.2 million and $126.2 million of unrecognized stock-based compensation expense related to unvested stock awards, expected to be recognized over a weighted average period of 2.4 years and 2.4 years, respectively.

The following table presents the stock activity for the three months ended March 31, 2022 and the total number of shares available for grant under our stock plans as of March 31, 2022:


						Plan Shares Available for Grant


Balances at December 31, 2021						24,146,784
Added to plan						8,384,460
Granted						(3,972,652)

Cancelled						261,572
Expired						(161,292)
Balances at March 31, 2022						28,658,872

During the three months ended March 31, 2022 and 2021, we recognized $2.5 million and $1.1 million of stock-based compensation expense for the 2018 Employee Stock Purchase Plan, respectively. We issued 420,689 shares in the three months ended March 31, 2022. During the three months ended March 31, 2022, we added an additional 2,055,792 shares and there were 4,179,771 shares available for issuance as of March 31, 2022. We issued 977,508 shares in the three months ended March 31, 2021.

As of March 31, 2022 and 2021, we had $9.1 million and $1.2 million of unrecognized stock-based compensation expense, expected to be recognized over a weighted average period of 1.1 years and 0.5 years, respectively.