Stockholders' Equity (Tables)

Total share-based compensation expense recorded for employees and non-employees, is as follows (in thousands):



	Years Ended December 31,
	2018		2017		2016
Cost of revenues	$	420	$	316	$	172
Research and development	17,055		14,333		14,771
Sales and marketing	6,703		5,007		6,124
General and administrative	27,852		18,703		20,718
Total share-based compensation expense	$	52,030	$	38,359	$	41,785

The following table summarizes the key assumptions used to determine the fair value of rights granted under the 2013 ESPP:



	Years Ended December 31,
	2018			2017			2016
Expected term (years)	0.50			0.50			0.50
Expected volatility	42.07%-44.97%			38.15%-45.57%			35.10%-75.74%
Dividend yield	—		%	—		%	—		%
Risk-free interest rate	2.09%-2.50%			1.04%-1.42%			0.38%-0.62%
Weighted-average grant-date fair value per share	$	7.14		$	3.55		$	1.79

The following table summarizes the key assumptions used to determine the fair value of our stock options granted:



	Year Ended December 31, 2016
Expected term (years)	5.50
Expected volatility	56.94		%
Dividend yield	—		%
Risk-free interest rate	1.43		%
Weighted-average grant-date fair value per share	$	2.58



	Options Outstanding
	Number of Options Outstanding		Weighted- Average Exercise Price per Share		Weighted-Average Remaining Contractual Term in Years	Aggregate Intrinsic Value
Balance at December 31, 2017	8,066,846		$	8.97	4.64	$	59,318,983
Exercised	(3,290,365	)	8.34
Balance at December 31, 2018	4,776,481		$	9.40	4.25	$	90,848,450

RSU and PSU Activity



	RSUs and PSUs Outstanding
	Number of RSUs and PSUs Outstanding		Weighted Average Grant Date Fair Value
Balance at December 31, 2017	14,335,115		$	6.78
Granted	3,691,552		21.67
Released	(5,616,838	)	6.64
Canceled	(1,605,021	)	7.28
Balance at December 31, 2018	10,804,808		$	11.87