STOCKHOLDERS’ EQUITY

Stock-Based Compensation Plans

We have stock-based compensation plans pursuant to which we have granted stock options and RSUs, including PSUs. The Company also has an ESPP for all eligible employees. As of March 31, 2015, there were a total of 44,040,580 shares of common stock available for grant under our stock-based compensation plans.

Employee Stock Options

The following table summarizes the weighted-average assumptions relating to our employee stock options:



	Three Months Ended
	March 31, 2015		March 31, 2014
Expected term in years	4.3		4.9
Volatility	40	%	45	%
Risk-free interest rate	1.5	%	1.7	%
Dividend rate	—	%	—	%

The following table summarizes the stock option activity and related information for the periods presented below (in thousands, except exercise prices and contractual life):



	Options Outstanding
	Number of Shares		Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Life (Years)	Aggregate Intrinsic Value
Balance—December 31, 2014	10,702		$	14.98
Granted	269		32.79
Forfeited	(73	)	24.59
Exercised	(1,931	)	10.49
Balance—March 31, 2015	8,967		$	16.41
Options vested and expected to vest—March 31, 2015	8,924		$	16.35	2.8	$	165,975
Options exercisable—March 31, 2015	7,589		$	14.71	2.4	$	153,634

The aggregate intrinsic value represents the pre-tax difference between the exercise price of stock options and the quoted market price of our common stock on March 31, 2015, for all in-the-money options. As of March 31, 2015, total compensation expense related to unvested stock options granted to employees but not yet recognized was $15.6 million, net of estimated forfeitures. This expense is expected to be amortized on a straight-line basis over a weighted-average period of 1.8 years.

Additional information related to our stock options is summarized below (in thousands, except per share amounts):



	Three Months Ended
	March 31, 2015		March 31, 2014
Weighted-average fair value per share granted	$	11.17	$	8.65
Intrinsic value of options exercised	41,003		15,321
Fair value of options vested	3,792		4,562

Restricted Stock Units

The following table summarizes the activity and related information for RSUs for the periods presented below (in thousands, except per share amounts):



	Restricted Stock Units Outstanding
	Number of Shares		Weighted-Average Grant-Date-Fair Value per Share
Balance—December 31, 2014	6,291		$	22.93
Granted	2,492		32.96
Forfeited	(198	)	26.36
Vested	(670	)	22.01
Balance—March 31, 2015	7,915		$	26.18
RSUs expected to vest—March 31, 2015	7,450		$	26.02

As of March 31, 2015, total compensation expense related to unvested RSUs that were granted to employees and non-employees, but not yet recognized, was $197.8 million, net of estimated forfeitures. This expense is expected to be amortized on a straight-line basis over a weighted-average vesting period of 3.1 years.

RSUs settle into shares of common stock upon vesting. Upon the vesting of the RSUs, we net-settle the RSUs and withhold a portion of the shares to satisfy minimum statutory employee withholding taxes. Total payment for the employees’ tax obligations to the taxing authorities is reflected as a financing activity within the condensed consolidated statements of cash flows.

The following summarizes the number and value of the shares withheld for employee taxes (in thousands):



	Three Months Ended
	March 31, 2015		March 31, 2014
Shares withheld for taxes	221		171
Amount withheld for taxes	$	6,600	$	3,633

Performance Stock Units

We have granted PSUs to certain of our executive officers and employees. PSUs granted to executive officers are based on the achievement of the market-based vesting conditions during the performance period, the final settlement of the PSUs will range between 0% and 150% of the target shares underlying the PSUs based on a specified objective formula approved by our Compensation Committee. The PSUs entitle our executive officers to receive a number of shares of our common stock based on the performance of our stock price over a two- or three-year period as compared to the NASDAQ Composite index for the same periods. PSUs granted to certain employees are based on the achievement of personal- and company-based performance vesting conditions during the performance period. The final settlement of these PSUs will range between 50% to 150% of the target shares underlying the PSUs based on specified objective formula approved by our Compensation Committee. The PSUs entitle the employees to receive a number of shares of our common stock based on a one year performance period, and vest equally in the second and third years.

The following table summarizes the weighted-average assumptions relating to our PSUs granted to our executive officers:



	Three Months Ended
	March 31, 2015		March 31, 2014
Expected term in years	3.0		3.0
Volatility	38	%	48	%
Risk-free interest rate	1.1	%	0.7	%
Dividend rate	—	%	—	%

The following table summarizes the activity and related information for PSUs for the periods presented below (in thousands, except per share amounts):



	Three Months Ended
	March 31, 2015		March 31, 2014
Shares granted to executive officers and employees	206		25
Weighted-average fair value per share granted	$	34.86	$	22.44

As of March 31, 2015, total compensation expense related to unvested PSUs that were granted to certain of our executive officers and employees, but not yet recognized, was $9.5 million, net of estimated forfeitures. This expense is expected to be amortized on a straight-line basis over a weighted-average vesting period of 2.5 years.

Employee Stock Purchase Plan

In determining the fair value of our ESPP, we use the Black-Scholes option pricing model that employs the following weighted-average assumptions:



	Three Months Ended
	March 31, 2015		March 31, 2014
Expected term in years	0.5		0.5
Volatility	28	%	37	%
Risk-free interest rate	0.1	%	0.1	%
Dividend rate	—	%	—	%

Additional information related to the ESPP is provided below (in thousands, except per share amounts):



	Three Months Ended
	March 31, 2015		March 31, 2014
Weighted-average fair value per share granted	$	7.56	$	5.76
Shares issued under the ESPP	427		424
Weighted-average price per share issued	$	21.34	$	17.18

Stock-based Compensation Expense

Stock-based compensation expense is included in costs and expenses as follows (in thousands):



	Three Months Ended
	March 31, 2015		March 31, 2014
Cost of product revenue	$	140	$	113
Cost of service revenue	1,632		1,329
Research and development	5,157		3,882
Sales and marketing	9,307		5,746
General and administrative	2,686		1,860
Total stock-based compensation expense	$	18,922	$	12,930

The following table summarizes stock-based compensation expense by award type (in thousands):



	Three Months Ended
	March 31, 2015		March 31, 2014
Stock options	$	3,455	$	4,692
RSUs (including PSUs)	14,292		7,363
ESPP	1,175		875
Total stock-based compensation expense	$	18,922	$	12,930

Total income tax benefit associated with stock-based compensation that is recognized in the condensed consolidated statements of operations is as follows (in thousands):



	Three Months Ended
	March 31, 2015		March 31, 2014
Income tax benefit associated with stock-based compensation	$	3,377	$	3,349

Share Repurchase Program

In December 2013, our Board of Directors (“Board”) authorized a Share Repurchase Program (“Program”) to repurchase up to $200.0 million of our outstanding common stock through December 31, 2014. Under the Program, share repurchases may be made by us from time to time in privately negotiated transactions or in open market transactions. The Program does not require us to purchase a minimum number of shares, and may be suspended, modified or discontinued at any time without prior notice. In October 2014, our Board extended the share repurchase authorization under the Program through December 31, 2015. During the three months ended March 31, 2015, there were no shares repurchased under the Program. As of March 31, 2015, $122.5 million remains available for future share repurchases under the Program.