Stockholders' Equity (Tables)

6 Months Ended

Jun. 30, 2016

Stock-based compensation expense is as follows (in millions):



	Three Months Ended June 30,				Six Months Ended June 30,
	2016		2015		2016		2015
Fulfillment	$	186	$	132	$	302	$	222
Marketing	80		50		136		84
Technology and content	419		319		737		552
General and administrative	83		62		137		111
Total stock-based compensation expense	$	768	$	563	$	1,312	$	969

The following table summarizes our restricted stock unit activity for the six months ended June 30, 2016 (in millions):



	Number of Units		Weighted-Average Grant-Date Fair Value
Outstanding as of December 31, 2015	18.9		$	362
Units granted	6.3		609
Units vested	(3.1	)	309
Units forfeited	(1.2	)	401
Outstanding as of June 30, 2016	20.9		$	443

Scheduled vesting for outstanding restricted stock units as of June 30, 2016, is as follows (in millions):



	Six Months Ended December 31,	Year Ended December 31,
	2016	2017	2018	2019	2020	Thereafter	Total
Scheduled vesting—restricted stock units	3.1	7.1	6.9	2.7	0.8	0.3	20.9