The use of psychological concepts to explain the behavior of individuals and groups that shape foreign policy is centuries old. Thucydides in his great History of the Peloponnesian War explored the impact of the fear of decline on leaders’ decisions to go to war. Barbara Tuchman in The Guns of August demonstrated how misperception and miscalculation by leaders in the summer of 1914 led to an accidental war that no leader wanted or expected. During and after World War II, political scientists began to draw systematically on psychological concepts to explain foreign policy behavior. Scholarship advanced when the International Society of Political Psychology was founded in 1978 along with a specialized journal, Political Psychology. Early scholarship focused on leaders’ personalities and their impact on the foreign policy choices they made, with special attention devoted to decisions to go to war or make peace. A second wave of scholarship drew on the work of cognitive psychologists who had identified heuristics and biases to explore the impact of the way leaders thought on the foreign policy decisions that they made and examined pairs of interacting leaders to explain spirals of escalation. Scholars mined cognitive psychology to explore decisions to cooperate or compete, the success and failure of deterrence and compellence, and bargaining and signaling behavior by leaders. A third wave of scholarship drew on psychological research on emotion and examined how the emotional states of leaders influenced foreign policy choices. Scholars moved beyond leaders to study elite and group attributes to explain foreign policy behavior. In doing so, they confronted the central problem of aggregation; cognition and emotion are embedded in the individual. When they move to explain group behavior, scholars deepened psychological concepts by adding a broader social dimension to the analysis. Research in the last decade situates feeling and thinking in a larger social and cultural context in a more contextualized explanation of foreign policy behavior. Research is increasingly multidisciplinary, drawing on neuroscience, evolutionary biology, evolutionary psychology, and behavioral economics to explain foreign policy behavior.