This paper is the first to perform a comprehensive estimation of employee stock option exercise behavior and option cost to firms. We develop a GMM-based methodology, robust to heteroskedasticity and correlation across exercises, for estimating the rate of voluntary option exercise as a function of the stock price path and of various firm and option holder characteristics. We use it to estimate an exercise function from a sample of 870,624 employee-option grants at 47 publicly-traded firms between 1980-2005, finding that volatility has a counterintuitive effect, and that men exercise faster than women. We also estimate the rate of employment termination, which determines forfeitures, cancellations and forced exercises. We use the estimated exercise and termination functions in a simulation based valuation model to analyze the effect of different firm and option holder characteristics on option value, and show that the true value of these options can differ substantially from values calculated using the usual FASB approximation.