This article starts from the assumption that emotions are inherently part of life in the international system, but that this is not as well reflected in the discipline of International Relations. The study of emotions can be incorporated more systematically into the discipline through more rigorous theorizing about how states—as main actors in world politics—experience and act on emotions. To do so, I draw on intergroup emotions theory, an emerging area of research in social psychology. This approach points out the process by which groups come to have emotional reactions, and from there how emotions generate intergroup perceptions and intergroup behavior—or foreign policies in the case of states. Understanding states-as-groups addresses many of the criticisms mainstream IR scholars direct toward the study of emotions, including how individual-level factors such as emotions matter for intergroup relationships.