We typically issue warrants to purchase shares of our common stock to investors as part of a financing transaction or in connection with services rendered by placement agents and consultants. Our outstanding warrants expire in July 2020. A summary of warrant activity is as follows:

(1)	Aggregate intrinsic value represents the difference between the exercise price of the warrant and the closing market price of our common stock on June 30, 2020, which was $30.25 per share.

Total intrinsic value of warrants exercised during the six months ended June 30, 2020 was $11.6 million.

Stock Options

We have five stock incentive plans: the 2002 Stock Incentive Plan (the “2002 Plan”), the 2009 Stock Incentive Plan (the “2009 Plan”), 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 The 2002 Plan, the 2009 Plan, the 2011 Plan and the 2015 Plan (the “Prior Plans”) have been superseded by the 2018 Plan.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. 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, 2020, the Company had 1,124,624 shares available for future awards under the 2018 Plan.

During the six months ended June 30, 2020, we granted stock options at exercise prices equal to the quoted market price of our common stock on the grant date. The fair value of each option grant was estimated on the date of grant using Black-Scholes with the following weighted average assumptions:


Expected life (years)	5.3 – 6.3
Risk-free interest rate	0.4% - 1.7	%
Volatility	69.8% – 77.3	%
Dividend yield	0	%

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. In April 2019, the Company amended its expected volatility assumption from using exclusively a historical volatility. The Company calculates its expected volatility assumption based on a blended volatility using an average of its historical and implied volatilities over the expected life of the stock-based award. The selection of the blended volatility assumption was based upon the Company’s assessment that blended volatility is more representative of the Company’s future stock price trends as it weighed in the longer term historical volatility with the near-term future implied volatility. We do not anticipate paying dividends on the common stock in the foreseeable future.

We recognize stock-based compensation expense over the vesting period using the straight-line method. Stock-based compensation expense is recognized only for those awards that vest. We account for the forfeitures of unvested awards as they occur.

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


	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2020		2019		2020		2019
Cost of revenues	$	82,893	$	101,999	$	151,489	$	164,752
General and administrative		1,442,467		1,343,984		2,517,636		2,284,123
Sales and marketing		601,319		443,253		969,647		782,328
Engineering and development		174,818		102,519		283,103		174,287
	$	2,301,497	$	1,991,755	$	3,921,875	$	3,405,490

A summary of stock option activity is as follows:


				Weighted-
		Weighted-		Average
		Average		Remaining	Aggregate
	Number of	Exercise		Contractual	Intrinsic
	Shares	Price/Share		Term (Years)	Value (1)
Outstanding — December 31, 2019	6,679,581	$	7.14
Granted (weighted-average fair value of $11.08 per share)	1,573,000		17.34
Exercised	(465,573)		6.43
Forfeited	(43,811)		10.12
Expired	(1,667)		22.68
Outstanding — June 30, 2020	7,741,530	$	9.23	7.0	$	162,705,100
Vested (exercisable) — June 30, 2020	5,811,748	$	6.75	6.2	$	136,551,100
Expected to vest after June 30, 2020 (unexercisable)	1,929,782	$	16.70	9.6	$	26,154,000

(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, 2020, which was $30.25 per share.

Total intrinsic value of options exercised during the six months ended June 30, 2020 was $7.1 million.

As of June 30, 2020, there was unrecognized compensation expense of $20.1 million related to unvested stock options, which we expect to recognize over a weighted average period of 3.1 years.