Our equity compensation plans provide for the grant of incentive and nonqualified stock options, as well as other stock-based awards including performance share units (“PSUs”), restricted stock awards (“RSAs”), restricted stock units (“RSUs”), and deferred stock units (“DSUs”). We also have an employee stock purchase plan (“ESPP”). We recognize stock-based compensation expense based on grant date award fair value. This cost is recognized over the period for which the employee is required to provide service in exchange for the award or the award performance period, except for expense relating to retirement-eligible employees that have not given their required notice, which is recognized on a pro-rata basis over the notice period prior to retirement. At March 31, 2021, there were 13.6 million shares available for grant under approved equity compensation plans.

Stock-based compensation expense was allocated in the condensed consolidated statements of comprehensive income as follows:

	Three Months Ended
	March 31,
(In thousands)	2021		2020
Cost of revenues	$	1,503	$	808
Operating expenses
Sales and marketing		1,482		846
Research and development		911		923
General and administrative		3,029		1,767
	$	6,925	$	4,344

	Three Months Ended
	March 31,
(In thousands)	2021		2020
Stock options	$	551	$	594
PSUs		2,084		696
RSUs		3,423		2,329
RSAs		106		127
ESPP		326		222
401(k) stock match		435		376
	$	6,925	$	4,344

As of March 31, 2021, there was $40.6 million of unrecognized stock-based compensation expense under our equity compensation plans, which is expected to be recognized on a straight-line basis over a weighted average period of 2.8 years.

	Three Months Ended March 31, 2021
				Weighted Average
	Options (#)			Exercise Price (per share)
Outstanding, beginning of period		944,886		$	36.71
Granted		34,395			110.42
Exercised		(87,658	)		31.97
Forfeited		(1,560	)		51.57
Outstanding, end of period		890,063			40.00

Of the total outstanding options at March 31, 2021, 0.7 million were exercisable. The outstanding and exercisable options had a weighted average exercise price of $33.82 per share and a weighted average remaining contractual life of 3.4 years.

The weighted average grant date fair value of options granted during the first three months of 2021 was $31.16 per share. This was estimated on the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions:

Volatility	35.3	%
Dividend yield	0	%
Life (in years)	4.0
Risk-free interest rate	0.4	%

Performance Share Units, Restricted Stock Units and Awards, and Deferred Stock Units

In each of the quarters ended March 31, 2021, 2020, 2019, and 2018 we granted PSU awards with a target performance level. These awards are earned based upon the Company’s total shareholder return as compared to an indexed total shareholder return over the course of a fiscal based three-year performance period, starting in the year of grant. Awards vest in the quarter following the conclusion of the performance period. In the three months ended March 31, 2021, PSU awards granted in 2018 vested at the maximum performance level and 0.1 million shares of common stock were issued.

	Three Months Ended March 31, 2021
				Weighted Average Grant
	#			Date Fair Value (per unit)
Outstanding, beginning of period		738,422		$	52.37
Granted		236,504			99.10
Vested and common stock issued		(286,888	)		38.58
Forfeited		(6,077	)		54.98
Outstanding, end of period		681,961			74.36

	Three Months Ended
	March 31,
(In thousands, except share data)	2021		2020
Amounts for shares purchased	$	105	$	87
Shares purchased		1,621		2,015

A total of 1.9 million shares of common stock are reserved for issuance under the ESPP as of March 31, 2021.

The fair value was estimated based on the market price of our common stock at the beginning of the offering period using the Black-Scholes option pricing model with the following assumptions:

Volatility	58.7	%
Dividend yield	0	%
Life (in years)	0.5
Risk-free interest rate	0.2	%