We recorded stock-based compensation expense related to grants of employee stock options, restricted stock units (“RSUs”) and performance and market-based awards in our consolidated statements of operations as follows (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2020		2019		2020		2019
Cost of revenue	$	566	$	407	$	1,548	$	1,088
Research and development		25,380		12,029		48,595		35,982
Sales and marketing		3,957		3,046		10,960		8,453
General and administrative		8,821		5,857		23,759		15,944
Total stock-based compensation expense	$	38,724	$	21,339	$	84,862	$	61,467

The following table shows stock option activity for the nine months ended September 30, 2020 (in thousands, except weighted-average exercise price and weighted-average contractual term):

	Outstanding Options
						Aggregate		Weighted-
				Weighted-Average		Intrinsic Value of		Average
				Exercise Price		Stock Options		Contractual Term
	Stock Options			(per option)		Outstanding		(in years)
Balance as of December 31, 2019		31,204		$	3.24	$	89,786		7.19
Granted		1,633			6.55
Forfeited, expired and cancelled		(19	)		3.37
Exercised		(2,435	)		2.93
Balance as of September 30, 2020		30,383		$	3.45	$	172,404		6.68

The following table presents the weighted-average grant date fair value and the related assumptions used to estimate the fair value of our stock options:

	Nine Months Ended September 30,
	2020
Expected term, in years		6
Risk-free interest rates		0.8	%
Expected volatility		36.0	%
Dividend yield		—
Weighted-average estimated fair value of options granted	$	2.35

The following table shows a summary of RSU activity for the nine months ended September 30, 2020, which includes performance and market-based awards (in thousands, except weighted-average grant date fair value):

	Outstanding RSUs

				Weighted-Average		Aggregate
				Grant Date Fair Value		Intrinsic Value of
	Shares			(per share)		Unvested RSUs
Unvested as of December 31, 2019		40,188		$	4.23	$	245,951
Granted		43,085			8.60
Vested		(13,650	)		4.04
Forfeited		(1,946	)		4.53
Unvested as of September 30, 2020		67,677		$	7.04	$	619,921

On March 15, 2020, the Company granted performance-based awards to certain executive officers. The number of shares earned will range from 0% to 120% of the target number of shares granted, depending on actual operating cash flows for the year ended December 31, 2020 relative to pre-established thresholds. Shares earned (if any) based on the level of achievement will vest over a period of four years following the grant date, with 25% vesting on the one year anniversary of the grant date and the remaining quarterly thereafter, subject to continued service by the executives. The target number of shares granted totaled 0.6 million.

The Company recognizes stock-based compensation expense for the Performance RSUs using the accelerated attribution method over the requisite service period. The amount of stock-based compensation is determined based on the probability of achievement of the pre-established thresholds at each reporting period. If necessary, at each reporting period, the Company records a cumulative catch-up adjustment to reflect any revised estimates regarding the probability of achievement.

On March 15, 2020, the Company granted market-based awards to its executive officers, with two separate prescribed measurement periods. Both prescribed measurement periods began on the grant date, with the first prescribed measurement period ending on December 31, 2021 (the “Two-Year Market RSUs”) and the second period ending on December 31, 2022 (the “Three-Year Market RSUs”). The number of RSUs earned (if any) will range from 0% to 150% of the target number of shares, depending on actual achievement of specific tiers for Zynga’s total shareholder return (“TSR”) relative to the TSR of companies comprising the S&P MidCap 400 Index over the prescribed measurement periods. If Zynga’s TSR is negative during any measurement period, the number of RSUs earned is capped at 100% of the target number of shares for that prescribed measurement period.

Any shares earned will vest in three equal annual increments, beginning on March 15 of the year following the conclusion of each prescribed measurement period, subject to continued service by the executives. The target number of shares totaled 2.2 million each for the Two-Year Market RSUs and the Three-Year Market RSUs, for a total of 4.4 million target shares.

The Company recognizes stock-based compensation expense for the Market RSUs using the accelerated attribution method over the requisite service period. The amount of stock-based compensation was determined based on the estimated grant date fair value, calculated using a Monte Carlo simulation that considers the probability of achievement of the specific tiers on each of the Market RSUs. Based upon the Monte Carlo simulation, the estimated per share grant date fair value of the Two-Year Market RSUs and Three-Year Market RSUs was $7.37 and $7.41, respectively. The significant assumptions used in estimating the grant date fair value of each award include an expected volatility of 27%, a risk-free interest rate ranging from 0.5% to 0.6% and an expected dividend yield of zero. The estimated grant date fair value is not subsequently revised to consider anticipated or actual achievement during the prescribed measurement periods.

In April 2018, a share repurchase program was authorized for up to $200.0 million of our outstanding Class A common stock (the “2018 Share Repurchase Program”). The timing and amount of any stock repurchase will be determined based on market conditions, share price and other factors. The program does not require us to repurchase any specific number of shares of our Class A common stock and may be modified, suspended or terminated at any time without notice. The 2018 Share Repurchase Program is authorized to remain in effect until April 2022 and will be funded from existing cash on hand or other sources of funding as the Company may determine to be appropriate. Share repurchases under these authorizations may be made through a variety of methods, which may include open market purchases, privately negotiated transactions, block trades, accelerated share repurchase transactions, purchases through 10b5-1 plans or by any combination of such methods. During the three and nine months ended September 30, 2020, no share repurchases were made under the 2018 Share Repurchase Program. As of September 30, 2020, we had $173.8 million remaining under the 2018 Share Repurchase Program.

All of our stock repurchases were made through open market purchases under Rule 10b5-1 plans and subsequently retired.