The following table summarizes stock options activity for the three months ended March 31, 2019 (in thousands):

	Number of Shares
Outstanding at December 31, 2018		3,603
Granted		57
Exercised		(28	)
Forfeited or expired		(178	)
Outstanding at March 31, 2019		3,454
Vested and exercisable at March 31, 2019		1,066

The following table summarizes activity for restricted stock units, or RSUs, and RSUs with performance conditions, or PSUs, for the three months ended March 31, 2019 (in thousands):

	Number of Shares
	RSUs		PSUs
Outstanding at December 31, 2018		32		—
Granted		3		242
Vested		—		—
Forfeited		—		—
Outstanding at March 31, 2019		35		242

PSUs granted during the three months ended March 31, 2019 were primarily related to 234,000 PSUs associated with a PSU bonus program that replaces what has historically been our annual cash-bonus program for our senior executives and other bonus-eligible employees. The number of PSUs that ultimately vest will depend on the extent to which we achieve certain 2019 financial metrics: 134,000 PSUs would vest at 100% achievement of such metrics, and 234,000 at 175% achievement. We record compensation expense each period based on our estimate of the most probable number of PSUs that will vest and recognize such expense over the related service period. During the three months ended March 31, 2019, stock-based compensation expense recognized for the PSUs was not material.

The following table presents the effects of stock-based compensation in our condensed consolidated statements of operations during the periods presented (in thousands):

	Three Months Ended March 31,
	2019		2018
Cost of revenue	$	144	$	83
Research and development expense		1,071		759
Sales and marketing expense		1,290		757
General and administrative expense		972		466
Total stock-based compensation expense	$	3,477	$	2,065