Share-based grants are designed to reward employees for their long-term contributions to us and provide incentives for them to remain with us.

As of March 31, 2021, options to purchase 557,148 shares of Class B common stock were outstanding with a weighted average exercise price of $23.75 per share. The 2002 Stock Plan has been canceled; however, it continues to govern outstanding grants under the 2002 Stock Plan.

As of March 31, 2021, stock options to purchase 7,797,591 shares of Class B common stock were outstanding with a weighted average exercise price of $27.24 per share and no shares were available for future grant. As of March 31, 2021, we had outstanding RSUs that may be settled for 71,000 shares of Class B common stock under the plan. The 2012 Equity Incentive Plan has been canceled; however, it continues to govern outstanding grants under the 2012 Equity Incentive Plan.

As of March 31, 2021, stock options to purchase 4,691,844 shares of Class A common stock were outstanding with a weighted average exercise price of $9.43 per share and 7,794,217 shares of outstanding RSUs that may be settled for Class A common stock which were granted pursuant to the 2018 Equity Incentive Plan ("2018 Plan"). As of March 31, 2021, we had 25,855,075 shares reserved for issuance under the 2018 Plan.

As of March 31, 2021, we had 3,512,465 shares reserved for future issuance under the 2018 Employee Stock Purchase Plan ("2018 ESPP"),

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,
	2021		2020

Cost of revenue	$	2,999	$	5,507
Research and development	4,908		6,096
Sales and marketing	4,085		3,890
General and administrative	5,218		7,526
	$	17,210	$	23,019

During the three months ended March 31, 2021 and 2020, we capitalized $0.8 million and $1.2 million, of stock-based compensation cost, respectively, into inventory and property, plant and equipment.

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, 2020	15,354,271	$	21.27	6.0	$	129,855

Exercised	(1,978,717)	26.90
Cancelled	(328,971)	10.64
Balances at March 31, 2021	13,046,583	20.69		6.0	106,497
Vested and expected to vest at March 31, 2021	12,772,269	20.92		6.0	101,777
Exercisable at March 31, 2021	8,755,992	25.52		5.0	36,250

Stock Options - During the three months ended March 31, 2021 and 2020, we recognized $4.0 million and $5.6 million of stock-based compensation costs for stock options, respectively. We did not grant options in the three months ended March 31, 2021 and 2020.

As of March 31, 2021 and 2020, we had unrecognized compensation costs related to unvested stock options of $16.8 million and $36.3 million, respectively. This cost is expected to be recognized over the remaining weighted-average period of 1.6 years and 2.4 years, respectively. Cash received from stock options exercised totaled $53.2 million and $0.7 million for the three months ended March 31, 2021 and 2020, respectively.


	Number of Awards Outstanding	Weighted Average Grant Date Fair Value









Unvested Balance at December 31, 2020	6,418,788	$	13.71
Granted	2,897,260	29.59
Vested	(1,140,502)	18.39
Forfeited	(310,328)	11.37
Unvested Balance at March 31, 2021	7,865,218	18.97

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, 2021 and 2020, we recognized $10.7 million and $13.2 million of stock-based compensation costs for stock awards, respectively.

As of March 31, 2021 and 2020, we had $126.2 million and $37.6 million of unrecognized stock-based compensation cost related to unvested stock awards, expected to be recognized over a weighted average period of 2.4 years and 1.3 years, respectively.

During 2020 and 2021, we granted PSUs to certain executive officers and employees that only vest upon the achievement of certain specific financial or operational performance criteria. Stock-based compensation costs associated with these PSUs is recognized over the service period as we evaluate the probability of the achievement of the performance conditions.

The following table presents the stock activity and the total number of shares available for grant under our stock plans as of March 31, 2021:


						Plan Shares Available for Grant













Balances at December 31, 2020						20,233,754
Added to plan						7,675,984
Granted						(2,628,268)
Exercised						—
Cancelled						639,299
Expired						(65,694)
Balances at March 31, 2021						25,855,075

During the three months ended March 31, 2021 and 2020, we recognized $1.1 million and $2.9 million of stock-based compensation costs for the 2018 ESPP, respectively. We issued 977,508 shares in the three months ended March 31, 2021. During the three months ended March 31, 2021, we added an additional 1,902,572 shares and there were 3,512,465 shares available for issuance as of March 31, 2021.

As of March 31, 2021 and 2020, we had $1.2 million and $5.5 million of unrecognized stock-based compensation costs, expected to be recognized over a weighted average period of 0.5 years and 0.8 years, respectively.

We used the following weighted-average assumptions in applying the Black-Scholes valuation model for determination of the 2018 ESPP share valuation:


	Three Months Ended March 31,
	2021	2020

Risk-free interest rate	0.06% - 0.13%	1.51%
Expected term (years)	0.5 - 2.0	0.5 - 2.0
Expected dividend yield	—	—
Expected volatility	95.0% - 109.0%	61.0%