For a detailed explanation of the Company's stock plans, refer to Note 14, Employee Benefits and Share-Based Compensation, of the Company's annual report on Form 10-K for the year ended December 31, 2020, filed with the SEC on February 24, 2021.

The following table sets forth the total share-based compensation expense recognized in the Company’s Condensed Consolidated Statements of Operations for the three months ended March 31, 2021 and 2020:


	Three Months Ended March 31,
	2021		2020

	(In thousands)
Cost of product and service revenues	$	1,937	$	1,770
Research and development	1,700		1,768
Selling, general, and administrative	8,135		7,121
Total share-based compensation expense	$	11,772	$	10,659

The following assumptions were used to value stock options and Employee Stock Purchase Plan (“ESPP”) shares granted pursuant to the Company’s equity incentive plans for the three months ended March 31, 2021 and 2020:


	Three Months Ended March 31,
	2021		2020

Stock options
Expected life, years	4.9		4.7
Expected volatility, %	30.1	%	33.6	%
Risk-free interest rate, %	0.6	%	1.4	%
Estimated forfeiture rate, %	7.9	%	5.7	%
Dividend yield, %	—	%	—	%


	Three Months Ended March 31,
	2021		2020

Employee stock purchase plan shares
Expected life, years	0.5 - 2.0		0.5 - 2.0
Expected volatility, %	27.4% - 53.5%		30.4% - 39.9%
Risk-free interest rate, %	0.1% - 2.6%		1.4% - 2.7%
Dividend yield, %	—	%	—	%

The following table summarizes the share option activity under the Company’s 2009 Equity Incentive Plan (the “2009 Plan”), as amended, during the three months ended March 31, 2021:


	Number of Shares	Weighted-Average Exercise Price		Weighted-Average Remaining Years	Aggregate Intrinsic Value

	(In thousands, except per share data)
Outstanding at December 31, 2020	3,932	$	62.50	7.8	$	226,160
Granted	150	125.80
Exercised	(196)	58.49
Expired	(6)	78.32
Forfeited	(60)	72.27
Outstanding at March 31, 2021	3,820	$	65.02	7.5	$	247,796
Exercisable at March 31, 2021	1,608	$	47.83	5.8	$	131,946
Vested and expected to vest at March 31, 2021 and thereafter	3,598	$	64.01	7.4	$	237,072

The weighted-average fair value per share of options granted during the three months ended March 31, 2021 and 2020 was $33.89 and $26.31, respectively. The intrinsic value of options exercised during the three months ended March 31, 2021 and 2020 was $14.0 million and $9.9 million, respectively.

As of March 31, 2021, total unrecognized compensation cost related to unvested stock options was $49.1 million, which is expected to be recognized over a weighted-average vesting period of 2.7 years.

For the three months ended March 31, 2021 and 2020, employees purchased approximately 156,000 and 217,000 shares of common stock, respectively, under the ESPP at a weighted-average price of $59.75 and $43.51, respectively. As of March 31, 2021, the unrecognized compensation cost related to the shares to be purchased under the ESPP was approximately $4.4 million and is expected to be recognized over a weighted-average period of 1.5 years.


	Number of Shares	Weighted-Average Grant Date Fair Value		Weighted-Average Remaining Years	Aggregate Intrinsic Value

	(In thousands, except per share data)
Restricted stock units
Outstanding at December 31, 2020	580	$	72.87	1.6	$	69,670
Granted (Awarded)	96	129.97
Vested (Released)	(37)	72.71
Forfeited	(17)	70.59
Outstanding and unvested at March 31, 2021	622	$	81.79	1.6	$	80,723

As of March 31, 2021, total unrecognized compensation cost related to RSUs was $44.8 million, which is expected to be recognized over the remaining weighted-average vesting period of 3.0 years.


	Number of Shares	Weighted-Average Grant Date Fair Value

	(In thousands, except per share data)
Restricted stock awards
Outstanding at December 31, 2020	21	$	68.11
Granted (Awarded)	—	—
Vested (Released)	—	—

Outstanding and unvested at March 31, 2021	21	$	68.11

As of March 31, 2021, total unrecognized compensation cost related to RSAs was $0.2 million, which is expected to be recognized over the remaining weighted-average vesting period of 0.2 years.


	Number of Shares	Weighted-Average Grant Date Fair Value Per Unit

	(In thousands, except per share data)
Outstanding at December 31, 2020	155	$	74.26
Granted	51	156.79
Vested	(17)	82.35
Forfeited	(6)	63.20
Outstanding and unvested at March 31, 2021	183	$	96.94

As of March 31, 2021, total unrecognized compensation cost related to PSUs was approximately $10.4 million, which is expected to be recognized over the remaining weighted-average vesting period of 1.5 years.


			Number of Shares

			(In thousands)
Share options outstanding			3,820
Non-vested restricted stock awards			826
Shares authorized for future issuance			872
ESPP shares available for future issuance			1,050
Total shares reserved for future issuance			6,568

On August 2, 2016, the Company's Board of Directors (the “Board”) authorized a stock repurchase program providing for the repurchase of up to $50.0 million of the Company’s common stock (the “2016 Repurchase Program”). The 2016 Repurchase Program is in addition to the stock repurchase program approved by the Board on November 4, 2014 providing for the repurchase of up to $50.0 million of the Company’s common stock (the “2014 Repurchase Program”). As of March 31, 2021, the maximum dollar value of shares that may yet be purchased under the two repurchase programs was $54.9 million. The stock repurchase programs do not obligate the Company to repurchase any specific number of shares, and the Company may terminate or suspend the repurchase programs at any time.