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). 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. Generally, our restricted stock units vest over four years and, (a) for employee new hire restricted stock unit grants, twenty-five percent vest one year from the vesting commencement date and continue to vest 1/16th per quarter thereafter; or (b) for employee refresh restricted stock unit grants, 1/16th per quarter vest from the vest commencement date schedule. As of January 31, 2021, 23,778,878 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. 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 January 31, 2021, 1,288,378 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, 2019		9,096,961		$	9.01		4.97	$	108,731
Options granted		577,082			19.89
Option exercised		(659,348	)		9.05
Options forfeited/cancelled		(242,110	)		17.63
Balance as of January 31, 2020		8,772,585		$	9.48		4.27	$	60,221
Options granted		31,666			12.48
Option exercised		(1,994,667	)		5.14
Options forfeited/cancelled		(192,547	)		10.73
Balance as of January 31, 2021		6,617,037		$	10.77		3.77	$	48,098
Vested and expected to vest as of January 31, 2021		6,554,892		$	10.68		3.74	$	48,092
Exercisable as of January 31, 2021		5,348,780		$	8.59		2.87	$	47,974

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, 2019 Options granted Option exercised Options forfeited/cancelled Balance as of January 31, 2020 Options granted Option exercised Options forfeited/cancelled Balance as of January 31, 2021 Vested and expected to vest as of January 31, 2021 Exercisable as of January 31, 2021 9,096,961 $9.01 4.97 $ 108,731 577,082 19.89 (659,34) 9.05 (242,110) 17.63 8,772,585 $ 9.48 4.27 $ 60,221 31,666 12.48 (1,994,667) 5.14 (192,547) 10.73 6,617,037 $ 10.773.77 $ 48,098 6,554,892 $ 10.68 3.74 $ 48,092 5,348,780 $ 8.59 2.87 $ 47,974

The aggregate intrinsic value of options vested and expected to vest and exercisable as of January 31, 2021 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 years ended January 31, 2021, 2020 and 2019 was $28.0 million, $5.9 million and $30.5 million, respectively. The aggregate estimated fair value of stock options granted to employees that vested during the years ended January 31, 2021, 2020 and 2019 was $2.3 million, $5.3 million and $6.7 million, respectively. The weighted-average grant date fair value of options granted to employees during the years ended January 31, 2021, 2020 and 2019 was $5.41, $8.00 and $8.26 per share, respectively.

As of January 31, 2021, there was $1.7 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 1.46 years.

To further align our stockholders’ interests with executive officers’ interests, the Compensation Committee of our board of directors approved and granted performance-based stock options with market-based performance goals under the 2015 Plan to certain executive officers, which are subject to both the achievement of the market-based performance goal established by the Compensation Committee and the continued employment of the participant. These performance-based stock options vest only to the extent that both the market-based performance goal and time-based condition are satisfied. As of January 31, 2021, the total outstanding balance of performance-based stock options was 1,375,000.

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. Of the total $1.7 million in unrecognized stock-based compensation expense for stock options as of January 31, 2021, $1.0 million related to outstanding performance-based stock options with market-based performance goals, which is expected to be recognized over a weighted-average period of 1.87 years.

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, 2019		18,098,707		$	19.35
Granted		12,436,586			18.81
Vested, net of shares withheld for employee payroll taxes		(4,166,907	)		19.92
Forfeited/cancelled		(4,560,279	)		19.77
Unvested balance - January 31, 2020		21,808,107		$	18.85
Granted		10,702,574			15.81
Vested, net of shares withheld for employee payroll taxes		(5,100,239	)		18.27
Forfeited/cancelled		(13,079,764	)		17.87
Unvested balance - January 31, 2021		14,330,678		$	17.68

er of Weighted- Restricted Average Stock Units Grant Date Outstanding Fair Value Unvested balance - January 31, 2019 18,098,707 $ 19.35 Granted 12,436,586 18.81 Vested, net of shares withheld for employee payroll taxes (4,166,907 ) 19.92 Forfeited/cancelled (4,560,279 ) 19.77 Unvested balance - January 31, 2020 21,808,107 $ 18.85 Granted 10,702,574 15.82 Vested, net of shares withheld for employee payroll taxes (5,100,239 ) 18.28 Forfeited/cancelled (13,079,764 ) 17.87 Unvested balance - January 31, 2021 14,330,678 $ 17.68

As of January 31, 2021, there was $236.2 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 2.58 years.

We use performance-based incentives for certain employees, including our named executive officers, to achieve our annual financial and operational objectives, while making progress towards our longer-term strategic and growth goals. Typically, near the beginning of each fiscal year, our Compensation Committee adopts the performance criteria and targets for the incentive compensation plan for that fiscal year, which identifies the plan participants, the performance measures and the associated target levels for each measure, and the potential payouts based on actual performance for the fiscal year.

In the first quarter of fiscal year 2020, our Compensation Committee adopted and approved the performance criteria and targets for fiscal year 2020 under our omnibus Executive Incentive Plan (the Fiscal 2020 Executive Bonus Plan). Based on a review of our actual achievement of pre-established corporate financial objectives and additional inputs from our Compensation Committee, the Fiscal 2020 Executive Bonus Plan was determined, settled and paid out in the first quarter of fiscal year 2021 in the form of fully vested restricted stock units. During the first quarter of fiscal year 2021, we recognized stock-based compensation expense related to the Fiscal 2020 Executive Bonus Plan in the amount of $1.5 million.

In the first quarter of fiscal year 2021, our Compensation Committee adopted and approved the performance criteria and targets for fiscal year 2021 under our omnibus Executive Plan (the Fiscal 2021 Executive Bonus Plan). The Fiscal 2021 Executive Bonus Plan provides opportunities for 100% equity incentive compensation payouts based on our actual achievement of pre-established corporate financial objectives, subject to review and a final approval by our Compensation Committee. During the year ended January 31, 2021, we recognized stock-based compensation expense related to the Fiscal 2021 Executive Bonus Plan in the amount of $8.3 million. The unrecognized compensation expense related to the ungranted and unvested Fiscal 2021 Executive Bonus Plan is $1.5 million, based on the expected performance against the pre-established corporate financial objectives as of January 31, 2021, which is expected to be recognized over a remaining weighted-average period of less than one year. The payouts of the Fiscal 2021 Executive Bonus Plan are expected to be made in the form of fully vested restricted stock units in the first quarter of fiscal year 2022.

During the first quarter of fiscal year 2021, the fair market value of our stock on the purchase date (i.e., March 15, 2020) was lower than the fair market value of our stock on the offering dates of our open offering periods. As a result, all open offering periods reset and the new lower price became the new offering price for a new 24 month offering period. These resets, including subsequent modifications, resulted in incremental fair value totaling $29.4 million during the year ended January 31, 2021, which is expected to be recognized over the remaining requisite service period. As of January 31, 2021, there was $27.2 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 consolidated statements of operations (in thousands):

	Year Ended January 31,
	2021		2020		2019
Cost of revenue	$	18,936	$	16,769	$	14,065
Research and development		61,145		62,565		45,189
Sales and marketing		42,015		38,030		36,864
General and administrative		32,196		28,624		23,178
Total stock-based compensation	$	154,292	$	145,988	$	119,296

Year Ended January 31, 2021 2020 2019 Cost of revenue $ 18,936 $ 16,769 $ 14,065 Research and development 61,145 62,565 45,189 Sales and marketing 42,015 38,030 36,864 General and administrative 32,196 28,624 23,178 Total stock-based compensation $ 154,292 $ 145,988 $ 119,296

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:

	Year Ended January 31,
	2021						2020						2019
Employee Stock Options
Expected term (in years)					5.8			5.5		–	5.8			5.5		–	5.8
Risk-free interest rate					0.6	%					1.8	%		2.8	%	–	3.1	%
Volatility					46	%					45	%					45	%
Dividend yield					0	%					0	%					0	%
Employee Stock Purchase Plan
Expected term (in years)		0.5		–	2.0			0.5		–	2.0			0.5		–	2.0
Risk-free interest rate		0.1	%	–	0.4	%		1.7	%	–	2.5	%		2.0	%	–	2.8	%
Volatility		44	%	–	54	%		34	%	–	55	%		37	%	–	50	%
Dividend yield					0	%					0	%					0	%

Year Ended January 31, 2021 2020 2019 Employee Stock Options Expected term (in years) 5.8 5.5 – 5.8 5.5 – 5.8 Risk-free interest rate 0.6% 1.8% 2.8% – 3.1% Volatility 46% 45% 45% Dividend yield 0% 0% 0% Employee Stock Purchase Plan Expected term (in years) 0.5 – 2.0 0.5 – 2.0 0.5 – 2.0 Risk-free interest rate 0.1% – 0.4% 1.7% – 2.5% 2.0% – 2.8% Volatility 44% – 54% 34% – 55% 37% – 50% Dividend yield 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 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.