Buyers make purchase decisions in a marketplace composed of dynamic and interacting product categories. In such an environment, demand for one product can depend directly and indirectly upon the marketing efforts involving "other products," i.e., products in different categories. Hence, buyer (and seller) decision-making can be influenced by what sellers of these other products are and are not doing, as well as by actions of direct competitors. In this paper, we argue for the relevance of these "other products" by considering a wide range of possible inter-category relationships. We offer a behavioral rationale for the existence of these effects and propose a taxonomy of inter-category relationships that do not fit neatly into the conventional product complement/substitute framework. This discussion allows us to identify a number of promising new research questions.*We thank David Stewart, former editor, and three anonymous reviewers for their many helpful and constructive comments. They have served to substantially improve this paper. Errors of commission and omission remain the responsibility of the authors.