We have three stock incentive plans: the 2011 Stock Incentive Plan (the “2011 Plan”), the 2015 Omnibus Equity Incentive Plan (the “2015 Plan”), and the 2018 Omnibus Equity Incentive Plan (the “2018 Plan”), (collectively, the “Plans”). The 2011 Plan and the 2015 Plan (the “Prior Plans”) have been superseded by the 2018 Plan. In May 2018, the stockholders approved the 2018 Plan for issuances up to an aggregate of 3,730,179 shares plus 1,269,821 shares that were authorized but unissued under the Prior Plans as of the effective date of the 2018 Plan and in April 2021 the stockholders approved an increase of 2,850,000 shares authorized under the 2018 Plan. The Prior Plans will remain in effect until all awards granted under such Prior Plans have been exercised, forfeited, cancelled, or have otherwise expired or terminated in accordance with the terms of such awards, but no awards will be made pursuant to the Prior Plans after the effectiveness of the 2018 Plan. As of June 30, 2021, the Company had 7,071,065 shares available for future awards under the 2018 Plan.

During the six months ended June 30, 2021, we granted stock options at exercise prices equal to the quoted market price of our common stock on the grant date, with the exception of certain officers of the Company who were granted options with an exercise price at a 10% premium to market price. The fair value of each option grant was estimated on the date of grant using Black-Scholes with the following weighted average assumptions:

The expected option life assumption is estimated based on the simplified method. Accordingly, the Company has utilized the average of the contractual term of the options and the weighted average vesting period for all options to calculate the expected option term. The risk-free interest rate assumption is based upon observed interest rates appropriate for the expected term of our employee stock options. The expected volatility is based on the average of the historical volatility and the implied volatility of our stock commensurate with the expected life of the stock-based award. We do not anticipate paying dividends on the common stock in the foreseeable future.

We recognize stock-based compensation cost on a straight-line basis over the vesting period. Stock-based compensation expense is recognized only for those awards that ultimately vest.

Total stock-based compensation expense related to all of our share-based payment awards is comprised of the following:

(1)	Aggregate intrinsic value represents the difference between the exercise price of the option and the closing market price of our common stock on June 30, 2021, which was $63.10 per share.

Total intrinsic value of options exercised during the six months ended June 30, 2021 was $19.6 million.

As of June 30, 2021, there was unrecognized compensation expense of $31.8 million related to unvested stock options, which we expect to recognize over a weighted average period of 3.1 years.

Restricted stock units

A summary of our restricted stock unit activity is as follows:


		Weighted Average
	Number of Restricted	Fair Value per
	Stock Units	Share
Outstanding – December 31, 2020	—	$	—
Granted	345,571		54.61
Forfeited	(6,359)		53.58
Share issuance	—		—
Outstanding June 30, 2021	339,212	$	54.63

For the three and six months ended June 30, 2021, we recorded stock-based compensation expense on our issued restricted stock units of $1.2 million and $1.5 million, respectively. There was no stock-based compensation expense on restricted stock units in 2020. As of June 30, 2021 there was unrecognized compensation expense of $17.1 million related to unvested restricted stock units, which we expect to recognize over a weighted average period of 3.6 years.