This article responds to key methodological and theoretical challenges posed by the literature on the role of ideas in international relations, especially the literature on ideas and the end of the Cold War. The article develops a theoretical framework that guides the analysis of the empirical articles that follow. It identifies explanatory strategies for the role of ideas and seeks to clarify key methodological issues in the study of ideas. The article defines terms, identifies several different relationships between ideational and material factors, and lays out a series of “tests” for evaluating the causal effect of various kinds of ideas and ideational mechanisms. It then seeks to clarify two primary issues: whether it is possible to draw a clearer line between the material and the ideational; and what is meant by “constitutive effects” and “constitutive explanation.” The article defends the notion of constitutive explanation and shows how both causal analysis and constitutive analysis are valid explanatory strategies for the role of ideas.