The Company's 2015 Long-Term Incentive Plan ("2015 Stock Plan") and 2021 Incentive Award Plan (“2021 Stock Plan” and, together, “Stock Plans”) permit the grant of restricted stock units (“RSUs”), stock options, and other stock-based awards to employees, non-employee directors, and consultants.

Stock option activity during the six months ended June 30, 2024 was not material. The following table summarizes the Company’s restricted stock unit activity during the six months ended June 30, 2024:


	RSUs
		Number of Shares (in millions)	Weighted-Average Grant-Date Fair Value
Outstanding at December 31, 2023		56	$	22.36
Granted		67	$	10.23
Exercised / Vested		(36)	$	16.35
Forfeited / Cancelled		(8)	$	20.95

Outstanding at June 30, 2024		79	$	15.03
Vested and expected to vest at June 30, 2024		79	$	15.03

The following table summarizes the Company’s stock-based compensation expense for the Stock Plans and 2021 Employee Stock Purchase Plan (“ESPP”) by line item in the Condensed Consolidated Statements of Operations (in millions):




	Three Months Ended June 30,				Six Months Ended June 30,
	2023		2024		2023		2024
Cost of revenues	$	23	$	17	$	41	$	40
Research and development	72		98		156		222
Selling, general, and administrative	86		79		167		165
Total stock-based compensation expense	$	181	$	194	$	364	$	427

As of June 30, 2024, the Company’s unrecognized stock-based compensation expense for unvested awards was $1,244 million, which is expected to be recognized over a weighted-average period of 5.2 years and 1.8 years for stock options and RSUs outstanding, respectively.