A summary of the status of stock option activity for the nine months ended September 30, 2019 is as follows (in thousands, except price data):



	Shares		Weighted-average exercise price per share
Outstanding at December 31, 2018	1,877		$	21.53
Granted	169		59.18
Exercised	(628	)	17.28
Forfeited	(3	)	42.29
Outstanding at September 30, 2019	1,415		$	27.89

A summary of the status of restricted stock unit activity for the nine months ended September 30, 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	255		59.45
Vested	(120	)	25.28
Forfeited	(4	)	46.83
Non-vested at September 30, 2019	807		$	40.52

During the nine months ended September 30, 2019, the Company issued 42,292 shares of common stock in connection with the Company’s employee stock purchase plan (the “ESPP”).

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 September 30,				Nine Months Ended September 30,
	Three Months Ended September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
Cost of sales	$	265	$	255	$	807	$	751
Research and development	573		539		1,682		1,732
Sales and marketing	827		686		2,850		2,199
General and administrative	1,459		1,295		4,745		4,508
Total stock-based compensation expense	$	3,124	$	2,775	$	10,084	$	9,190

As of September 30, 2019, total unrecognized compensation expense was $26.4 million, which is expected to be recognized over a weighted-average period of approximately 1.95 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.



	Nine Months Ended September 30,
	2019			2018
Risk-free interest rate	2.51		%	2.49		%
Expected option life (in years)	5.7			6.3
Volatility rate	39		%	36		%
Dividend rate	0		%	0		%
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 nine months ended September 30, 2019 and 2018 was $59.45 and $49.45, respectively.