This book focuses on foreign policy decision-making from the viewpoint of psychology. Psychology is always present in human decision-making, constituted by its structural determinants but also playing its own agency-level constitutive and causal roles, and therefore it should be taken into account in any analysis of foreign policy decisions. The book analyses a wide variety of prominent psychological approaches, such as bounded rationality, prospect theory, belief systems, cognitive biases, emotions, personality theories and trust to the study of foreign policy, identifying their achievements and added value as well as their limitations from a comparative perspective. Understanding how leaders in world politics act requires us to consider recent advances in neuroscience, psychology and behavioral economics. As a whole, the book aims at better integrating various psychological theories into the study of international relations and foreign policy analysis, as partial explanations themselves but also as facets of more comprehensive theories. It also discusses practical lessons that the psychological approaches offer since ignoring psychology can be costly: decision-makers need to be able reflect on their own decision-making process as well as the perspectives of the others. Paying attention to the psychological factors in international relations is necessary for better understanding the microfoundations upon which such agency is based.