In January 2015, our board of directors adopted the 2015 Equity Incentive Plan (2015 Plan), which became effective prior to the completion of our initial public offering (IPO). A total of 12,200,000 shares of Class A common stock was initially reserved for issuance pursuant to future awards under the 2015 Plan. On the first day of each fiscal year, shares available for issuance are increased based on the provisions of the 2015 Plan. Any shares subject to outstanding awards under our 2006 Equity Incentive Plan (2006 Plan) or 2011 Equity Incentive Plan (2011 Plan) that are cancelled or repurchased subsequent to the 2015 Plan’s effective date are returned to the pool of shares reserved for issuance under the 2015 Plan. Awards granted under the 2015 Plan may be (i) incentive stock options, (ii) nonstatutory stock options, (iii) restricted stock units, (iv) restricted stock awards or (v) stock appreciation rights, as determined by our board of directors at the time of grant. Twenty-five percent of each grant of stock options and restricted stock units generally vest one year from the vesting commencement date and continue to vest (a) in the case of options, 1/48^th per month thereafter, and (b) in the case of restricted stock units, 1/16th per quarter thereafter. As of October 31, 2018, 18,518,116 shares were reserved for future issuance under the 2015 Plan.

In January 2015, our board of directors adopted the 2015 Employee Stock Purchase Plan (2015 ESPP), which became effective prior to the completion of our IPO. A total of 2,500,000 shares of Class A common stock was initially reserved for issuance under the 2015 ESPP. On the first day of each fiscal year, shares available for issuance are increased based on the provisions of the 2015 ESPP. The 2015 ESPP allows eligible employees to purchase shares of our Class A common stock at a discount of up to 15% through payroll deductions of their eligible compensation, subject to any plan limitations. The 2015 ESPP provides for 24-month offering periods beginning March 16 and September 16 of each year, and each offering period consists of four six-month purchase periods.

On each purchase date, eligible employees may purchase our stock at a price per share equal to 85% of the lesser of (1) the fair market value of our stock on the offering date or (2) the fair market value of our stock on the purchase date. In the event the price is lower on the last day of any purchase price period, in addition to using that price as the basis for that purchase period, the offering period resets and the new lower price becomes the new offering price for a new 24 month offering period. As of October 31, 2018, 1,957,913 shares were reserved for future issuance under the 2015 ESPP.

The following table summarizes the stock option activity under the equity incentive plans and related information:

	Shares Subject to Options Outstanding					Weighted-Average
				Weighted-		Remaining
				Average Exercise		Contractual Life		Aggregate
	Shares			Price		(Years)		Intrinsic Value
								(in thousands)
Balance as of January 31, 2018		10,843,120		$	8.32		5.74	$	150,922
Options granted		717,658			20.78
Options exercised		(1,745,370	)		8.58
Options forfeited/cancelled		(461,801	)		14.10
Balance as of October 31, 2018		9,353,607		$	8.94		5.20	$	87,143
Vested and expected to vest as of October 31, 2018		9,281,847		$	8.87		5.17	$	87,071
Exercisable as of October 31, 2018		7,788,714		$	7.04		4.50	$	85,578

The aggregate intrinsic value of options vested and expected to vest and exercisable as of October 31, 2018 is calculated based on the difference between the exercise price and the current fair value of our common stock. The aggregate intrinsic value of exercised options for the nine months ended October 31, 2018 and 2017 was $27.5 million and $18.8 million, respectively. The aggregate estimated fair value of stock options granted to employees that vested for the nine months ended October 31, 2018 and 2017 was $5.6 million and $7.2 million, respectively. The weighted-average grant date fair value of options granted to employees during the nine months ended October 31, 2018 and 2017 was $8.24 and $7.04 per share, respectively.

As of October 31, 2018, there was $9.0 million of unrecognized stock-based compensation expense related to outstanding stock options granted to employees that is expected to be recognized over a weighted-average period of 2.59 years. Out of the total unrecognized stock-based compensation expense, as of October 31, 2018, $3.8 million related to outstanding performance-based stock option, based on the probable market-based performance goals and service condition at that date, which is expected to be recognized over a weighted-average period of 3.29 years.

In April 2017, the compensation committee of our board of directors approved and granted 475,000 performance-based stock options under the 2015 Plan to certain executive officers where vesting is subject to both the continued employment of the participant and the achievement of market-based performance goals established by the compensation committee. Subject to both the continued employment of the participant and the achievement of market-based performance goals established by the compensation committee, 25% of the performance-based options vest one year from the vesting commencement date, and 1/48th continue to vest each month thereafter. The total amount of compensation expense recognized is based on the number of shares that we determine are probable of vesting and is recognized over the vesting term of the awards. Of the 475,000 performance-based stock options, 250,000 were forfeited in connection with a participant’s resignation of employment during the three months ended October 31, 2017. During fiscal year 2019, the market-based performance goals were met. The performance-based stock options are vesting based on the continued employment of the participant. The grant date fair value of these awards was determined using a Monte Carlo valuation model and the related stock-based compensation expense is recognized based on an accelerated attribution method.

In April 2018, the compensation committee of our board of directors approved and granted 650,000 performance-based stock options under the 2015 Plan to certain executive officers where vesting is subject to both the continued employment of the participant and the achievement of market-based performance goals established by the compensation committee. Subject to both the continued employment of the participant and the achievement of market-based performance goals established by the compensation committee, 25% of the performance-based options vest one year from the vesting commencement date, and 1/48th continue to vest each month thereafter. The total amount of compensation expense recognized is based on the number of shares that we determine are probable of vesting and is recognized over the vesting term of the awards. As of October 31, 2018, the market-based performance goals were not met. The grant date fair value of these awards was determined using a Monte Carlo valuation model and the related stock-based compensation expense is recognized based on an accelerated attribution method.

The following table summarizes the restricted stock unit activity under the equity incentive plans and related information:

	Number of			Weighted-
	Restricted			Average
	Stock Units			Grant Date
	Outstanding			Fair Value
Unvested balance - January 31, 2018		14,619,252		$	16.42
Granted		9,298,770			22.58
Vested, net of shares withheld for employee payroll taxes		(2,656,081	)		16.51
Forfeited/cancelled		(2,922,723	)		17.41
Unvested balance - October 31, 2018		18,339,218		$	19.38

As of October 31, 2018, there was $273.1 million of unrecognized stock-based compensation expense related to outstanding restricted stock units granted to employees that is expected to be recognized over a weighted-average period of 3.08 years.

As of October 31, 2018, there was $16.8 million of unrecognized stock-based compensation expense related to the 2015 ESPP that is expected to be recognized over the remaining term of the respective offering periods.

The following table summarizes the components of stock-based compensation expense recognized in the condensed consolidated statements of operations (in thousands):

	Three Months Ended				Nine Months Ended
	October 31,				October 31,
	2018		2017		2018		2017
Cost of revenue	$	3,598	$	2,814	$	10,280	$	7,945
Research and development		12,043		9,705		33,668		28,419
Sales and marketing		9,708		8,208		27,701		23,882
General and administrative		6,441		4,796		17,437		12,290
Total stock-based compensation	$	31,790	$	25,523	$	89,086	$	72,536

We estimated the fair value of employee stock options and 2015 ESPP purchase rights using a Black-Scholes option pricing model with the following assumptions.

	Three Months Ended										Nine Months Ended
	October 31,										October 31,
	2018					2017					2018					2017
Employee Stock Options
Expected term (in years)					5.8					6.1		5.5	-		5.8		5.5	-		6.1
Risk-free interest rate				3.1%					2.0%		2.8%		-	3.1%		1.8%		-	2.1%
Volatility				45%					39%					45%		38%		-	40%
Dividend yield				0%					0%					0%					0%
Employee Stock Purchase Plan
Expected term (in years)		0.5	-		2.0		0.5	-		2.0		0.5	-		2.0		0.5	-		2.0
Risk-free interest rate	2.3%		-	2.8%		1.2%		-	1.4%		2.0%		-	2.8%		0.9%		-	1.4%
Volatility	40%		-	48%		29%		-	40%		37%		-	50%		28%		-	43%
Dividend yield				0%					0%					0%					0%

The assumptions used in the Black-Scholes option pricing model were determined as follows:

Fair Value of Common Stock. We use the market closing price for our Class A common stock as reported on the New York Stock Exchange to determine the fair value of our common stock at each grant date.

Expected Term. The expected term represents the period that our share-based awards are expected to be outstanding. The expected term assumptions were determined based on the vesting terms, exercise terms and contractual lives of the options and 2015 ESPP purchase rights.

Expected Volatility. We estimate the expected volatility of the stock option grants and 2015 ESPP purchase rights based on the historical volatility of our Class A common stock over a period equivalent to the expected term of the stock option grants and 2015 ESPP purchase rights, respectively.

Risk-free Interest Rate. The risk-free rate that we use is based on the implied yield available on U.S. Treasury zero-coupon issues with remaining terms similar to the expected term on the stock options and 2015 ESPP purchase rights.

Dividend Yield. We have never declared or paid any cash dividends and do not plan to pay cash dividends in the foreseeable future, and, therefore, use an expected dividend yield of zero.