The term “international regime” was originally used to describe formal agreements between states, but the concept has since evolved after going through considerable critique and reformulation. A universal agreement on the precise nature or elements of a regime has remained elusive, despite a general consensus on the definition. Nevertheless, the concept of regime offers a unique opportunity to better understand international relationships by underscoring the importance of specific attributes of international, multinational, and nongovernmental groups, sets of behavioral or epistemic practices, and processes of learning. As a heuristic device, regime theory helps to explain the rise of complex interaction between states, organizations, corporations, and other institutions as well as the potential for ideas or behavior to shape the international system. Regime theory has supplemented traditional explanations of international order, including hegemonic stability theory or neorealism, by explaining the emergence of cooperation and organization within what would traditionally be considered anarchical or highly unpredictable conditions. Common approaches to regime theory include realism, neoliberalism, cognitivism, and constructivism. Part of the strength of regime theory is that it has remained an elastic concept and has been used to analyze a huge diversity of issues, with many promising results. Regime theory should continue highlighting both the ideational and material dimensions of organization and bringing together positivist, inductive, and critical approaches to understanding power, interest, and identity so as to generate a series of new conversations or trajectories for exploring the creation of international order.