Despite the proliferation of loyalty programs in a wide range of categories, there is little empirical research that focuses on the measurement of such programs. The key to measuring the influence of loyalty programs is that they operate as dynamic incentive schemes by providing benefits based on cumulative purchasing over time. As such, loyalty programs encourage consumers to shift from myopic or single-period decision making to dynamic or multiple-period decision making. In this study, the author models customers' response to a loyalty program under the assumption that purchases represent the sequential choices of customers who are solving a dynamic optimization problem. The author estimates the theoretical model using a discrete-choice dynamic programming formulation. The author evaluates a specific loyalty program with data from an online merchant that specializes in grocery and drugstore items. Through simulation and policy experiments, it is possible to evaluate and compare the long-term effects of the loyalty program and other marketing instruments (e.g., e-mail coupons, fulfillment rates, shipping fees) on customer retention. Empirical results and policy experiments suggest that the loyalty program under study is successful in increasing annual purchasing for a substantial proportion of customers.