Summary: A company’s assortment of products and corresponding inventory levels are constrained by available resources, such as production capacity, storage space, and capital to acquire the inventory. Thus, customers may not always be able to find a most preferred product at the time of purchase; this unsatisfied demand is often substituted with an alternative. In the extant literature, there have been an increasing number of studies that consider product substitution when planning product assortment, inventory, and capacity, in conjunction with pricing. In this paper we classify the literature on the planning of substitutable products published in the major OM and marketing journals during the past thirty years (1974–2013) and present a comprehensive taxonomy of the literature. One criterion is adopted to discuss modeling objectives, and three major criteria are provided to define the nature of product substitution, including substitution mechanism, substitution decision maker, and direction of substitutability. We also identify research gaps to provide guidance for related research in the future.