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 IPO in 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, 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, 2021 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, 2020	229	28,734	$	12.87	4.97	$	1,104
Granted	—	945	$	14.68
Exercised	(48)	(6,177)	$	12.30
Canceled and forfeited	(2)	(300)	$	18.24
As of June 30, 2021	179	23,202	$	13.03	4.02	$	867
Vested and expected to vest as of June 30, 2021	138	17,773	$	8.39	3.45	$	752
Exercisable as of June 30, 2021	138	17,773	$	8.39	3.45	$	752


	Number of Shares	Weighted-Average Grant-Date Fair Value per Share
Unvested and outstanding as of December 31, 2020	83,736	$	34.17
Granted	24,573	$	58.35
Vested	(18,467)	$	36.67
Canceled and forfeited	(14,956)	$	36.45
Unvested and outstanding as of June 30, 2021	74,886	$	41.07

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 for the three and six months ended June 30, 2020 and 2021 (in millions):


	Three Months Ended June 30,				Six Months Ended June 30,
	2020		2021		2020		2021
Operations and support	$	11	$	38	$	36	$	65
Sales and marketing	10		19		24		42
Research and development	72		149		239		282
General and administrative	38		66		109		164
Total	$	131	$	272	$	408	$	553

As of June 30, 2021, there was $2.6 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 3.02 years.