STOCKHOLDERS’ EQUITY

Preferred Stock

We have authorized 500 million shares of $0.01 par value preferred stock. No preferred stock was outstanding for any year presented.

Common Stock

Common shares outstanding plus shares underlying outstanding stock awards totaled 497 million, 504 million, and 507 million, as of December 31, 2016, 2017, and 2018. These totals include all vested and unvested stock awards outstanding, including those awards we estimate will be forfeited.

Stock Repurchase Activity

In February 2016, the Board of Directors authorized a program to repurchase up to $5.0 billion of our common stock, with no fixed expiration. There were no repurchases of common stock in 2016, 2017, or 2018.

Stock Award Plans

Employees vest in restricted stock unit awards and stock options over the corresponding service term, generally between two and five years.

Stock Award Activity

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



	Year Ended December 31,
	2016		2017		2018
Cost of sales	$	16	$	47	$	73
Fulfillment	657		911		1,121
Marketing	323		511		769
Technology and content	1,664		2,305		2,888
General and administrative	315		441		567
Total stock-based compensation expense (1)	$	2,975	$	4,215	$	5,418

___________________


(1)	The related tax benefits were $907 million, $860 million, and $1.1 billion for 2016, 2017, and 2018. In 2017 and 2018, the tax benefit reflects the permanent reduction in the U.S. statutory corporate tax rate from 35% to 21%.

The following table summarizes our restricted stock unit activity (in millions):



	Number of Units		Weighted Average Grant-Date Fair Value
Outstanding as of January 1, 2016	18.9		$	362
Units granted	9.3		660
Units vested	(6.1	)	321
Units forfeited	(2.3	)	440
Outstanding as of December 31, 2016	19.8		506
Units granted	8.9		946
Units vested	(6.8	)	400
Units forfeited	(1.8	)	649
Outstanding as of December 31, 2017	20.1		725
Units granted	5.0		1,522
Units vested	(7.1	)	578
Units forfeited	(2.1	)	862
Outstanding as of December 31, 2018	15.9		$	1,024

Scheduled vesting for outstanding restricted stock units as of December 31, 2018, is as follows (in millions):



	Year Ended
	2019	2020	2021	2022	2023	Thereafter	Total
Scheduled vesting — restricted stock units	6.9	5.6	2.4	0.8	0.1	0.1	15.9

As of December 31, 2018, there was $6.6 billion of net unrecognized compensation cost related to unvested stock-based compensation arrangements. This compensation is recognized on an accelerated basis with approximately half of the compensation expected to be expensed in the next twelve months, and has a weighted-average recognition period of 1.1 years. The estimated forfeiture rate as of December 31, 2016, 2017, and 2018 was 28%, 28%, and 27%. Changes in our estimates and assumptions relating to forfeitures may cause us to realize material changes in stock-based compensation expense in the future.

During 2016, 2017, and 2018, the fair value of restricted stock units that vested was $4.3 billion, $6.8 billion, and $11.4 billion.

Common Stock Available for Future Issuance

As of December 31, 2018, common stock available for future issuance to employees is 113 million shares.