Stockholders’ Equity

Equity Incentive Plans

In May 2015, the Company’s board of directors and stockholders adopted and approved the 2015 Equity Incentive Plan (the “2015 Plan”). The 2015 Plan became effective on June 16, 2015 and serves as the successor to the Amended and Restated 2007 Stock Plan (the “2007 Plan”). The Company ceased granting awards under the 2007 Plan, and any outstanding stock options and RSUs granted under the 2007 Plan would remain subject to the terms of the 2007 Plan. As of March 30, 2019, 24.4 million shares of Class A common stock were reserved and available for future issuance under the 2015 Plan.

Stock Options

Stock option activity under the equity incentive plans was as follows (in thousands, except per share amounts):



	Stock Options Outstanding
	Number of Shares Subject to Stock Options		Weighted– Average Exercise Price		Aggregate Intrinsic Value ⁽¹⁾
Balance—December 31, 2018	16,263		$	3.00
Granted	—		—
Exercised	(479	)	$	1.99
Forfeited or canceled	(10	)	$	3.03
Balance—March 30, 2019	15,774		$	3.03	$	50,435

Stock options vested and expected to vest—March 30, 2019	15,774		$	3.03	$	50,435
Stock options exercisable—March 30, 2019	15,181		$	2.88	$	50,010

⁽¹⁾ The aggregate intrinsic values of stock options outstanding, exercisable, vested and expected to vest as of March 30, 2019 were calculated as the difference between the exercise price of the stock options and the fair value of the Class A common stock of $5.92 as of March 30, 2019.

Restricted Stock Units

RSU activity under the equity incentive plans was as follows (in thousands, except per share amounts):



	RSUs Outstanding		Weighted- Average Grant Date Fair Value
Unvested balance—December 31, 2018	18,376		$	6.69
Granted	10,021		$	5.83
Vested	(2,983	)	$	7.38
Forfeited or canceled	(1,412	)	$	6.69
Unvested balance—March 30, 2019	24,002		$	6.25

In March 2019, the Company issued 0.5 million shares of market-based awards that vest based upon the achievement of a specified stock price. Market conditions were factored into the grant date fair value using a Monte Carlo valuation model, which utilized multiple input variables to determine the probability of the Company achieving the specified stock price targets. Stock-based compensation expense related to these awards will be recognized over the requisite service period regardless of whether the market condition is satisfied, provided that the requisite service period has been completed.

Employee Stock Purchase Plan

In May 2015, the Company’s board of directors adopted the 2015 Employee Stock Purchase Plan (the “2015 ESPP”), which became effective on June 17, 2015. The 2015 ESPP allows eligible employees to purchase shares of the Company’s Class A common stock through payroll deductions at a price per share equal to 85% of the lesser of the fair market value of the Company’s Class A common stock (i) on the first trading day of the applicable offering period and (ii) the last trading day of each purchase period in the applicable offering period. Except for the initial offering period, the 2015 ESPP provides for 6-month offering periods beginning in May and November of each year.

Stock-Based Compensation Expense

Total stock-based compensation expense recognized was as follows (in thousands):



	Three Months Ended
	March 30, 2019		March 31, 2018
Cost of revenue	$	1,430	$	1,098
Research and development	11,988		14,671
Sales and marketing	3,138		3,447
General and administrative	3,988		4,425
Total stock-based compensation expense	$	20,544	$	23,641

As of March 30, 2019, the total unrecognized stock-based compensation expense related to unvested stock options and RSUs was $138.8 million, which the Company expects to recognize over an estimated weighted average period of 2.2 years.