The Company maintains long-term incentive plans for employee, non-employee members of our Board of Directors and consultants. The plans allows us to grant equity-based compensation awards, including stock options, stock appreciation rights, performance share units, restricted stock units, restricted stock awards, or a combination of awards (collectively, share-based awards).

The Company accounts for share-based payments through the measurement and recognition of compensation expense for share-based payment awards made to employees and directors of the Company, including stock options and restricted shares. The Company also issues share based payments in the form of common stock warrants to non-employees.

The following table presents share-based payment expense for the three months ended March 31, 2021 and 2020.

As of March 31, 2021, the Company had approximately $2.6 million of unamortized share-based compensation for option awards and restricted stock awards, which is expected to be recognized over a weighted average period of approximately 1.75 years. As of March 31, 2021, the Company also had approximately $3.6 million of unamortized share-based compensation for common stock warrants issued to consultants, which is expected to be recognized over a weighted average period of 2.75 years.

Restricted Stock

The Company issues shares of restricted stock to eligible employees, which are subject to forfeiture until the end of an applicable vesting period. The awards generally vest on the second or third anniversary of the date of grant, subject to the employee’s continuing employment as of that date.

Restricted stock activity for the three months ended March 31, 2021 is presented in the following table:

The table below summarizes all option activity under all plans during the three months ended March 31, 2021:

A summary of the status of the Company’s outstanding stock purchase warrants for the three months ended March 31, 2021 is as follows: