For a detailed explanation of the Company's stock plans, refer to Note 14, Employee Benefits and Share-Based Compensation, of the audited Consolidated Financial Statements and accompanying Notes included in 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 and six months ended June 30, 2021 and 2020:


	Three Months Ended June 30,				Six Months Ended June 30,
	2021		2020		2021		2020

	(In thousands)
Cost of product and service revenues	$	2,044	$	2,130	$	3,981	$	3,900
Research and development	1,731		1,854		3,431		3,622
Selling, general, and administrative	9,264		7,367		17,399		14,488
Total share-based compensation expense	$	13,039	$	11,351	$	24,811	$	22,010


	Three Months Ended June 30,				Six Months Ended June 30,
	2021		2020		2021		2020
Expected life, years	0.5 - 2.0		0.5 - 2.0		0.5 - 2.0		0.5 - 2.0
Expected volatility, %	27.4% - 53.5%		30.4% - 39.9%		27.4% - 53.5%		30.4% - 39.9%
Risk-free interest rate, %	0.1% - 2.6%		1.4% - 2.7%		0.1% - 2.6%		1.4% - 2.7%
Dividend yield, %	—	%	—	%	—	%	—	%

For the six months ended June 30, 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 June 30, 2021, the unrecognized compensation cost related to the shares to be purchased under the ESPP was approximately $2.9 million and is expected to be recognized over a weighted-average period of 1.3 years.

The following assumptions were used to value stock options granted pursuant to the Company’s 2009 Equity Incentive Plan (as amended, the “2009 Plan”) for the three months ended June 30, 2020, and the six months ended June 30, 2021 and 2020. There were no stock options granted during the three months ended June 30, 2021.


	Three Months Ended June 30,		Six Months Ended June 30,
	2020		2021		2020
Expected life, years	4.7		4.9		4.7
Expected volatility, %	42.2	%	30.1	%	37.9	%
Risk-free interest rate, %	0.5	%	0.6	%	0.9	%
Estimated forfeiture rate, %	5.7	%	7.9	%	5.7	%
Dividend yield, %	—	%	—	%	—	%


	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	(395)	58.20
Expired	(8)	68.18
Forfeited	(120)	73.01
Outstanding at June 30, 2021	3,559	$	65.28	7.3	$	306,648
Exercisable at June 30, 2021	1,634	$	49.45	5.9	$	166,665
Vested and expected to vest at June 30, 2021 and thereafter	3,381	$	64.38	7.3	$	294,384

The weighted-average fair value per share of options granted during the three months ended June 30, 2020 was $26.03, and the weighted-average fair value per share of options granted during the six months ended June 30, 2021 and 2020 was $33.89 and $26.21, respectively. The intrinsic value of options exercised during the three months ended June 30, 2021 and 2020 was $16.8 million and $2.5 million, respectively, and during the six months ended June 30, 2021 and 2020 was $30.8 million and $12.4 million, respectively.

As of June 30, 2021, total unrecognized compensation cost related to unvested stock options was $43.8 million, which is expected to be recognized over a weighted-average vesting period of 2.6 years.


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

	(In thousands, except per share data)

Outstanding at December 31, 2020	580	$	72.87	1.6	$	69,670
Granted (Awarded)	165	134.00
Vested (Released)	(86)	75.56
Forfeited	(36)	71.92
Outstanding and unvested at June 30, 2021	623	$	88.75	1.5	$	94,403

As of June 30, 2021, total unrecognized compensation cost related to RSUs was $48.2 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)

Outstanding at December 31, 2020	21	$	68.11
Granted (Awarded)	10	135.86
Vested (Released)	(21)	68.11

Outstanding and unvested at June 30, 2021	10	$	135.86

As of June 30, 2021, total unrecognized compensation cost related to RSAs was $1.2 million, which is expected to be recognized over the remaining weighted-average vesting period of 0.9 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	(41)	70.60
Forfeited	(6)	63.20
Outstanding and unvested at June 30, 2021	159	$	102.21

As of June 30, 2021, total unrecognized compensation cost related to PSUs was approximately $8.6 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			3,559
Non-vested restricted stock awards			792
Shares authorized for future issuance			1,902
ESPP shares available for future issuance			1,050
Total shares reserved for future issuance			7,303

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 June 30, 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.