A distinct subfield of international relations, IPE, has emerged over the last thirty years, largely in the pages of International Organization. IPE began with the study of international political economy, but over time its boundaries have been set more by a series of theoretical debates than by subject matter. These debates have been organized around points of contestation between specific research programs, reflecting fundamental differences among the generic theoretical orientations in which these research programs are embedded. The fate of specific research programs has depended on their ability to specify cause and effect relationships and to operationalize relevant variables. Scholarship in IPE has become more sophisticated both methodologically and theoretically, and many of its insights have been incorporated into policy discussions. Past points of contestation, including those between realism and its liberal challengers and between various conceptions of domestic structure and international relations, help us to understand recent debates between rationalism and constructivism.