This article questions the centrality of interest-based explanation in political science. Through an examination of the “turn to ideas” undertaken in the past decade by rationalist and nonrationalist scholars in both comparative politics and international relations, it seeks to make three points. First, interests are far from the unproblematic and ever-ready explanatory instruments we assume them to be. Second, the ideational turn of historical institutionalism and constructivist international relations theory marks a substantive theoretical shift in the field precisely because it problematizes notions of action that take interest as given. Third, such scholarship emerged from, and in reaction to, the inherent limits of rationalist treatments of interests and ideas. That it did so suggests that progress in the discipline may be more dialectic—rather than linear or paradigmatic—than we realize.