We maintain four equity compensation plans that provide for the issuance of shares of our common stock to our officers and other employees, directors, and consultants: the 2010 Stock Plan (the “2010 Plan”), the 2013 Equity Incentive Plan (the “2013 Plan”), the 2019 Equity Incentive Plan (the “2019 Plan”), and the 2019 Employee Stock Purchase Plan (the “ESPP”), which have all been approved by stockholders. Following our initial public offering (“IPO”) in May 2019, we have only issued awards under the 2019 Plan and the ESPP, and no additional awards will be granted under the 2010 and 2013 Plans. These plans provide for the issuance of incentive stock options (“ISOs”), nonqualified stock options (“NSOs”), stock appreciation rights (“SARs”), restricted stock awards, restricted stock units (“RSUs”), performance-based awards, and other awards (that are based in whole or in part by reference to our common stock).

A summary of stock option and SAR activity for the six months ended June 30, 2023 is as follows (in millions, except share amounts which are reflected in thousands, per share amounts, and years):


	SARs Outstanding Number of SARs	Options Outstanding Number of Shares	Weighted-Average Exercise Price Per Share		Weighted-Average Remaining Contractual Life (in years)	Aggregate Intrinsic Value
As of December 31, 2022	153	20,039	$	13.90	3.47	$	279
Granted	—	403	$	32.99
Exercised	(2)	(3,060)	$	4.71
Canceled and forfeited	—	(132)	$	6.90
As of June 30, 2023	151	17,250	$	16.00	3.13	$	476
Vested and expected to vest as of June 30, 2023	146	12,602	$	10.99	2.74	$	410
Exercisable as of June 30, 2023	146	12,602	$	10.99	2.74	$	410


	Number of Shares	Weighted-Average Grant-Date Fair Value per Share
Unvested and outstanding as of December 31, 2022	98,167	$	34.70
Granted	56,795	$	33.07
Vested	(26,859)	$	34.64
Canceled and forfeited	(8,927)	$	34.78
Unvested and outstanding as of June 30, 2023	119,176	$	33.91

Stock-based compensation expense is allocated based on the cost center to which the award holder belongs. The following table summarizes total stock-based compensation expense by function (in millions):


	Three Months Ended June 30,				Six Months Ended June 30,
	2022		2023		2022		2023
Operations and support	$	40	$	45	$	73	$	83
Sales and marketing	28		26		50		50
Research and development	277		317		473		607
General and administrative	125		116		233		234
Total	$	470	$	504	$	829	$	974

As of June 30, 2023, there was $4.0 billion of unamortized compensation costs related to all unvested awards. The unamortized compensation costs are expected to be recognized over a weighted-average period of approximately 2.73 years.