The following table summarizes the outstanding stock option activity during the periods indicated (shares in thousands):

				Weighted average
	Number of shares			Exercise price		Remaining contractual term (years)
Balance at December 31, 2019		1,600		$	9.98
Granted		341			10.14
Exercised		—			1.90
Expired/Forfeited		(17	)		9.70
Balance at March 31, 2020		1,924			10.01		8.1

Vested and exercisable at March 31, 2020		892			8.74		7.0
Vested and expected to vest at March 31, 2020		1,924			10.01		8.1

The weighted average grant date fair value of options granted during the three months ended March 31, 2020, and for the year ended December 31, 2019, was $4.30 and $4.93, respectively. For fully vested stock options, the aggregate intrinsic value as of March 31, 2020, and December 31, 2019, was $1.1 million and $2.3 million, respectively. For stock options expected to vest, the aggregate intrinsic value as of March 31, 2020 and December 31, 2019 was $15,000 and $300,000, respectively.

At March 31, 2020, and December 31, 2019, there was $4.2 million and $3.2 million, respectively, of total unrecognized compensation cost related to unvested stock options granted under the Company’s equity plans. That cost is expected to be recognized over the next four years and is based on the date the options were granted.

The following table summarizes the Company's Restricted Stock Unit (RSU) activity for the three months ended March 31, 2020 (shares in thousands):

	Restricted stock units			Weighted average grant date fair value
Non-vested balance at December 31, 2019		80		$	11.43
Grants		151			10.17
Vested and released		(28	)		11.17
Forfeitures		—			—
Non-vested balance at March 31, 2020		203			10.53

As of March 31, 2020, there was $2.0 million of total unrecognized compensation cost related to unvested RSUs having a weighted average remaining contractual term of 2.1 years.

The Company maintains the Employee Stock Purchase Plan (ESPP) that provides employees an opportunity to purchase common stock through payroll deductions. The ESPP is implemented through consecutive 6-month offering periods commencing on March 1 and September 1 of each year. The first offering period under the ESPP commenced on March 1, 2019. The purchase price is set at 85% of the fair market value of the Company's common stock on either the first or last trading day of the offering period, whichever is lower, and annual contributions are limited to the lower of 20% of an employee's eligible compensation or such other limits as apply under Section 423 of the Internal Revenue Code for such plans such as the ESPP. The ESPP is intended to qualify as an employee stock purchase plan for purposes of Section 423 of the Internal Revenue Code.

Based on the 15% discount and the fair value of the option feature of the ESPP, it is considered compensatory. Compensation expense is calculated using the fair value of the employees’ purchase rights under the Black-Scholes model. The Company currently uses authorized and unissued shares to satisfy share award exercises.

The Company received proceeds of $99,000 from the issuance of 15,000 shares under the ESPP in February 2020.