11. Stock-based Compensation

The Company may grant qualified and non-qualified stock options, RSUs, common stock and other stock-based awards under various plans to directors, officers, employees and other individuals. Stock options are granted at exercise prices of not less than the fair market value of the Company’s common stock on the date of grant. Additionally, the Company has an Employee Stock Purchase Plan (“ESPP”) that allows employees to purchase Company shares at a purchase price equal to 85% of the lower of the fair market value of the Company’s common stock on the first trading day of the offering period or on the last day of the offering period.

Stock Options

Stock options generally vest monthly over three years for employees and one year for directors. Stock options held by employees who qualify for retirement age (defined as employees that are a minimum of 55 years of age and the sum of their age plus years of full-time employment with the Company exceeds 70 years) vest on the earlier of scheduled vest date or the date of retirement.

The fair value of the stock options granted during the three and six months ended June 30, 2020 and 2019, was estimated based on the following weighted average assumptions:

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Stock option activity for the six months ended June 30, 2020, was as follows:

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The aggregate intrinsic value of stock options exercised during the six months ended June 30, 2020 and 2019 was $40,000 and $0.2 million, respectively.

The weighted-average grant-date fair value per share of the options granted during the six months ended June 30, 2020 and 2019 was $16.22 and $11.37, respectively.

As of June 30, 2020, $4.0 million of total unrecognized compensation expense related to stock options is expected to be recognized over a weighted average period of 1.78 years.

Performance-Based Stock Options

Stock-based compensation expense associated with the corporate performance-based stock options is recognized if the performance condition is considered probable of achievement using management’s best estimates. In 2017, the Company granted performance-based stock options with vesting criteria related to performance in 2017, 2018, and 2019. In 2019, the Company had 41,250 shares remaining related to outstanding performance-based stock options with a grant date fair value of $0.2 million that had vesting criteria based solely on the achievement of fiscal year 2019 corporate goals as set by the Compensation Committee of the Company’s Board of Directors. For the year ended December 31, 2019, the Company determined that all remaining performance criteria were achieved and therefore the related expense of $0.2 million was recognized for the year ended December 31, 2019. After December 31, 2019, no performance-based stock options were outstanding and there was no unrecognized compensation cost related to performance-based stock options.

Stock-based Compensation Expense

The following table shows total stock-based compensation expense for stock options and ESPP in the condensed consolidated statements of operations and comprehensive loss (in thousands):

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