Stockholders’ Equity

Issuances of Common Stock

A summary of the status of stock option activity for the three months ended March 31, 2019 is as follows (in thousands, except price data):



	Number of Shares		Weighted- average exercise price per share
Outstanding at December 31, 2018	1,877		$	21.53
Granted	168		59.18
Exercised	(355	)	17.77
Cancelled	(2	)	37.37
Outstanding at March 31, 2019	1,688		$	26.06

A summary of the status of restricted stock unit activity for the three months ended March 31, 2019 is as follows (in thousands, except price data):



	Shares		Weighted-average grant date fair value
Non-vested at December 31, 2018	676		$	30.75
Granted	220		59.45
Vested	(68	)	21.45
Forfeited	(2	)	42.51
Non-vested at March 31, 2019	826		$	39.11

During the three months ended March 31, 2019, the Company issued 21,600 shares of common stock in connection with the Company’s employee stock purchase plan (the “ESPP”).

Stock-Based Compensation

The expense related to the Company’s stock-based compensation plans included in the accompanying Consolidated Statements of Income was as follows (in thousands):



	Three months ended March 31,
	Three months ended March 31,
	2019		2018
Cost of sales	$	280	$	231
Research and development	565		592
Sales and marketing	1,119		796
General and administrative	1,624		1,317
Total stock-based compensation expense	$	3,588	$	2,936

As of March 31, 2019, total unrecognized compensation expense was $30.3 million, which is expected to be recognized over a weighted-average period of approximately 2.4 years.

The estimated fair value of each stock option was determined on the date of grant using the Black-Scholes option valuation model with the following weighted-average assumptions for the option grants.



	Three months ended March 31,
	2019			2018
Risk-free interest rate	2.51		%	2.49		%
Expected option life (in years)	5.68			6.29
Volatility rate	39		%	36		%
Dividend rate	—		%	—		%
Weighted-average grant date fair value	$	23.67		$	18.76

The fair value of RSUs is determined based on the closing market price of the Company’s common stock on the grant date. The weighted-average fair value of RSUs granted during the three months ended March 31, 2019 and 2018 was $59.45 and $46.48, respectively.