We have issued performance-based stock options and performance-based RSUs, the vesting of which is generally contingent upon the achievement of certain performance criteria related to our operating performance, as well as successful and timely development and market acceptance of future product introductions. In addition, certain of the performance RSUs have additional service requirements subsequent to the achievement of the performance criteria. Compensation expense is recognized over the requisite service period, which is defined as the longest explicit, implicit or derived service period based on management’s estimate of the probability of the performance criteria being satisfied, adjusted at each balance sheet date. For both service-based and performance-based RSUs, we account for forfeitures as they occur as a reduction to stock-based compensation expense and additional paid-in-capital.

For performance-based options with a vesting schedule based entirely on the attainment of both performance and market conditions, stock-based compensation expense is recognized for each pair of performance and market conditions over the longer of the expected achievement period of the performance and market conditions, beginning at the point in time that the relevant performance condition is considered probable of achievement. The fair value of such awards is estimated on the grant date using Monte Carlo simulations.

On May 24, 2018, our stockholders approved the Board of Directors’ grant of 6,365,856 stock option awards to Patrick W. Smith, our CEO (the “CEO Performance Award”). The CEO Performance Award consists of 12 vesting tranches with a vesting schedule based entirely on the attainment of both operational goals (performance conditions) and market capitalization goals (market conditions), assuming continued employment either as the CEO or as both Executive Chairman and Chief Product Officer and service through each vesting date. Each of the 12 vesting tranches of the CEO Performance Award have a 10-year contractual term and will vest upon certification by the Board of Directors that both (i) the market capitalization goal for such tranche, which begins at $2.5 billion for the first tranche and increases by increments of $1.0 billion thereafter, and (ii) any one of the following eight operational goals focused on revenue or eight operational goals focused on Adjusted EBITDA have been met for the previous four consecutive fiscal quarters. Adjusted EBITDA for purposes of the CEO Performance Award ("Adjusted EBITDA (CEO Performance Award)") is defined as net income (loss) attributable to common stockholders before interest expense, investment interest income, provision (benefit) for income taxes, depreciation and amortization, and stock-based compensation expense.

(1)	In connection with the business acquisition that was completed during the three months ended June 30, 2018, the revenue goals were adjusted for the acquiree’s Target Revenue, as defined in the CEO Performance Award agreement.

As of March 31, 2020, the following operational goals were considered probable of achievement:

●

Total revenue of $710.1 million, $860.1 million, and $1,010.1 million; and

●

Adjusted EBITDA (CEO Performance Award) of $125.0 million, $155.0 million, $175.0 million, $190.0 million, $200.0 million, and $210.0 million.

The first two market capitalization goals have been achieved as of March 31, 2020. However, none of the stock options granted under the CEO Performance Award have vested thus far as the operational goals have not yet been achieved as of March 31, 2020. As there are nine operational goals considered probable of achievement, we recorded stock-based compensation expense of $44.3 million related to the CEO Performance Award from the Grant Date through March 31, 2020. The number of stock options that would vest related to the nine tranches is approximately 4.8 million shares.

As of March 31, 2020, we had $147.3 million of total unrecognized stock-based compensation expense for the performance goals that were considered probable of achievement, which will be recognized over a weighted-average period of 5.79 years. As of March 31, 2020, we had unrecognized stock-based compensation expense of $54.4 million for the performance goals that were considered not probable of achievement.

eXponential Stock Performance Plan

On February 12, 2019, our shareholders approved the 2019 Stock Incentive Plan (the “2019 Plan”), which was adopted by the Board of Directors to reserve a sufficient number of shares to facilitate our eXponential Stock Performance Plan (“XSPP”) and grants of eXponential Stock Units (“XSUs”) under the plan. Initial awards under the plan were granted in January 2019, with additional employee awards granted since that date. During the three months ended March 31, 2020, we granted an additional 43 thousand XSUs.

The XSUs are grants of RSUs, each with a term of approximately nine years, that vest in 12 equal tranches. Each of the 12 tranches will vest upon certification by the Compensation Committee of the Board of Directors that both (i) the market capitalization goal for such tranche, which begins at $2.5 billion for the first tranche and increases by increments of $1.0 billion thereafter, and (ii) any one of eight operational goals focused on revenue or eight operational goals focused on Adjusted EBITDA (CEO Performance Award) have been met for the previous four consecutive fiscal quarters.

The first two market capitalization goals have been achieved as of March 31, 2020. However, none of the XSU tranches have vested thus far as the operational goals have not yet been achieved as of March 31, 2020. As there are nine operational goals considered probable of achievement, we recorded stock-based compensation expense of $23.2 million related to the XSU awards from their respective grant dates through March 31, 2020. The number of XSU awards that would vest related to the nine tranches is approximately 4.1 million shares.

As of March 31, 2020, we had $131.8 million of total unrecognized stock-based compensation expense for the performance goals that were considered probable of achievement, which will be recognized over a weighted-average period of 5.65 years. As of March 31, 2020, we had unrecognized stock-based compensation expense of $35.5 million for the performance goals that were considered not probable of achievement.

Restricted Stock Units

The following table summarizes RSU activity for the three months ended March 31, 2020 (number of units and aggregate intrinsic value in thousands):


	Number of	Weighted Average		Aggregate
	Units	Grant-Date Fair Value		Intrinsic Value
Units outstanding, beginning of year	1,249	$	45.47
Granted	78		69.19
Released	(219)		35.49
Forfeited	(34)		50.82
Units outstanding, end of period	1,074		49.05	$	75,985

Aggregate intrinsic value represents our closing stock price on the last trading day of the period, which was $70.77 per share, multiplied by the number of RSUs outstanding. As of March 31, 2020, there was $41.4 million in unrecognized compensation costs related to RSUs under our stock plans for shares that are expected to vest. We expect to recognize the cost related to the RSUs over a weighted average period of 2.09 years. RSUs are released when vesting requirements are met.

During the three months ended March 31, 2020, we granted 0.1 million service-based RSUs. Certain RSUs that vested in the three months ended March 31, 2020 were net-share settled such that we withheld shares to cover the employees’ tax obligation for the applicable income and other employment taxes, and remitted the cash to the appropriate taxing authorities. Total shares withheld related to RSUs were approximately 53 thousand and had a value of $4.0 million on their respective vesting dates as determined by the closing stock price on such dates. Payments for the employees’ tax obligations are reflected as a financing activity within the condensed consolidated statements of cash flows. We record a liability for the tax withholding to be paid by us as a reduction to additional paid-in capital.

Performance Stock Units

The following table summarizes Performance Stock Units (“PSUs“) activity for the three months ended March 31, 2020 (number of units and aggregate intrinsic value in thousands):


	Number of	Weighted Average		Aggregate
	Units	Grant-Date Fair Value		Intrinsic Value
Units outstanding, beginning of year	6,033	$	34.47
Granted	102		42.41
Released	(158)		24.88
Forfeited	(188)		34.97
Units outstanding, end of period	5,789		34.86	$	409,657

Aggregate intrinsic value represents our closing stock price on the last trading day of the period, which was $70.77 per share, multiplied by the number of PSUs outstanding. As of March 31, 2020, there was $137.1 million in unrecognized compensation costs related to PSUs under our stock plans for shares that are expected to vest. We expect to recognize the cost related to the PSUs over a weighted average period of 5.52 years. PSUs are released when vesting requirements are met.

During the three months ended March 31, 2020, we granted 0.1 million PSUs, including approximately 43 thousand XSUs. As of March 31, 2020, the performance criteria had been met for approximately 0.1 million of the 5.8 million PSUs outstanding. Certain of the PSUs outstanding as of March 31, 2020 can vest with a range of shares earned being between 0% and 200% of the targeted shares granted, depending on the final achievement of pre-determined performance criteria as of the vesting date. The amount of PSUs included in the table above related to such grants is the

target level. The maximum additional number of PSUs that could be earned is 0.2 million, which are not included in the table above.

Certain PSUs that vested in the three months ended March 31, 2020 were net-share settled such that we withheld shares to cover the employees’ tax obligation for the applicable income and other employment taxes, and remitted the cash to the appropriate taxing authorities. Total shares withheld related to PSUs were approximately 15 thousand and had a value of $1.2 million on their respective vesting dates as determined by the closing stock price on such dates. Payments for the employees’ tax obligations are reflected as a financing activity within the condensed consolidated statements of cash flows. We record a liability for the tax withholding to be paid by us as a reduction to additional paid-in capital.

Stock Option Activity

The following table summarizes stock option activity for the three months ended March 31, 2020 (number of units and aggregate intrinsic value in thousands):


				Weighted
		Weighted		Average
	Number	Average		Remaining
	of	Exercise		Contractual	Aggregate
	Options	Price		Life (years)	Intrinsic Value
Options outstanding, beginning of year	6,431	$	28.34
Granted	—		—
Exercised	(6)		4.74
Expired / terminated	—		—
Options outstanding, end of period	6,425		28.36	7.84	$	272,512
Options exercisable, end of period	59		4.50	0.67		3,936

Aggregate intrinsic value represents the difference between the exercise price of the underlying stock option awards and the closing market price of our common stock of $70.77 on March 31, 2020. The intrinsic value of options exercised for the three months ended March 31, 2020 and 2019 was $0.3 million and $1.0 million, respectively. As of March 31, 2020, total options outstanding included 6.4 million unvested performance-based stock options. Of this total, 4.8 million options relate to tranches of the CEO Performance Award considered probable of achievement.

Stock-based Compensation Expense

The following table summarizes the composition of stock-based compensation expense for the three months ended March 31, 2020 and 2019 (in thousands):


	Three Months Ended March 31,
	2020		2019
Cost of products sold and services delivered	$	590	$	226
Sales, general and administrative expenses		14,970		4,681
Research and development expenses		4,635		2,998
Total stock-based compensation expense	$	20,195	$	7,905

Stock Incentive Plan

In February 2019, our shareholders approved the 2019 Plan authorizing an additional 6.0 million shares, plus remaining available shares under prior plans, for issuance under the new plan. Combined with the legacy stock incentive plans, there are 2.1 million shares available for grant as of March 31, 2020.

Stock Repurchase Plan

In February 2016, our Board of Directors authorized a stock repurchase program to acquire up to $50.0 million of our outstanding common stock subject to stock market conditions and corporate considerations. During the three months ended March 31, 2020 and 2019, no common shares were purchased under the program. As of March 31, 2020, $16.3 million remains available under the plan for future purchases. Any future purchases will be discretionary.