The following table summarizes stock-based compensation expense related to stock options, performance stock units (“PSUs”), restricted stock units (“RSUs”) and restricted stock:

	Three Months Ended						Six Months Ended
	June 30,						June 30,
(In thousands)	2021			2020			2021			2020
Stock-based compensation expense included in cost of revenue	$	125		$	87		$	256		$	202
Selling, general and administrative expense		1,098			971			2,196			2,046
Research and development expense		585			597			1,163			1,198
Stock-based compensation expense included in operating expenses		1,683			1,568			3,359			3,244
Total stock-based compensation expense		1,808			1,655			3,615			3,446
Tax benefit for expense associated with stock options, PSUs, RSUs and restricted stock		(452	)		(394	)		(883	)		(821	)
Total stock-based compensation expense, net of tax	$	1,356		$	1,261		$	2,732		$	2,625

The following table summarizes PSUs, RSUs and restricted stock outstanding as of December 31, 2020 and June 30, 2021 and the changes that occurred during the six months ended June 30, 2021:

	Number of Shares (in thousands)			Weighted Avg. Grant Date Fair Value (per share)
Unvested PSUs, RSUs and restricted stock outstanding, December 31, 2020		1,846		$	11.49
PSUs, RSUs and restricted stock granted		357		$	16.73
PSUs, RSUs and restricted stock vested		(12	)	$	11.76
PSUs, RSUs and restricted stock forfeited		(47	)	$	11.58
Unvested PSUs, RSUs and restricted stock outstanding, June 30, 2021		2,144		$	12.41

During each of the six month periods ended June 30, 2021 and 2020, the Company granted 0.3 million performance-based PSUs to its executive officers and certain employees. The grant-date fair value of these performance-based awards was based on the closing price of the Company’s stock on the date of grant. These awards vest over two-year and three-year periods, respectively, subject to the grantee’s continued employment, with the ability to earn shares in a range of 0% to 142.8% of the awarded number of PSUs based on

the achievement of defined performance targets. Equity-based compensation expense with respect to these awards may be adjusted over the vesting period to reflect the probability of achievement of performance targets defined in the award agreements.

The fair value of RSUs and restricted stock is equal to the closing price of our stock on the date of grant. The fair value of PSUs with market conditions is calculated using a Monte Carlo simulation valuation method.

As of June 30, 2021, total unrecognized compensation expense related to non-vested market-based PSUs, RSUs and restricted stock was approximately $13.2 million, which will be recognized over the remaining weighted-average period of 2.5 years. In addition, there was $7.5 million of unrecognized compensation expense related to unvested 2020 and 2021 performance-based PSUs, which will be recognized over the remaining requisite service period of 1.5 years if achievement of the performance obligation becomes probable. Unrecognized compensation expense will be adjusted for actual forfeitures.

As of June 30, 2021, 3.5 million shares were available for issuance under stockholder-approved equity plans.

The following table summarizes stock options outstanding as of December 31, 2020 and June 30, 2021 and the changes that occurred during the six months ended June 30, 2021:

	Number of Stock Options (in thousands)			Weighted Avg. Exercise Price (per share)		Weighted Avg. Remaining Contractual Life (in years)		Aggregate Intrinsic Value (in thousands)
Stock options outstanding, December 31, 2020		2,718		$	21.17		2.9	$	—
Stock options exercised		(215	)	$	16.53				588
Stock options expired		(117	)	$	23.37				17
Stock options outstanding, June 30, 2021		2,386		$	21.47		2.4	$	4,721
Stock options exercisable, June 30, 2021		2,386		$	21.47		2.4	$	4,721

As of June 30, 2021, there was no unrecognized compensation expense related to stock options as all awards vested in prior periods.

There were no stock options granted during the six months ended June 30, 2021 or 2020. All of the options were previously issued at exercise prices that approximated fair market value at the date of grant.

The aggregate intrinsic value of stock options represents the total pre-tax intrinsic value (the difference between ADTRAN’s closing stock price on the last trading day of the quarter and the exercise price, multiplied by the number of in-the-money options) that would have been received by the option holders had all option holders exercised their options on June 30, 2021. The amount of aggregate intrinsic value was $4.7 million as of June 30, 2021 and will change based on the fair market value of ADTRAN’s stock. The total pre-tax intrinsic value of options exercised during the six months ended June 30, 2021 was $0.6 million.