This entry examines the concept of rationality in science and, more specifically, psychology. It begins from the premise that rationality is as old as Western philosophy yet remains crucial for understanding the scientific status of psychology. The central problem is how to conceive rationality without reducing it either to an instrumental means–ends calculus or to an abstract, timeless ideal of logical correctness. The entry pursues a dual aim: first, to provide an analytical clarification of what is meant by rationality when discussing methods, decisions, and practices in psychology; and second, to show how such an understanding shapes ways of conceiving psychology as both a science and a profession. The analysis combines conceptual inquiry with a historical–epistemological overview. It examines the assumptions of the Cartesian turn and modern science and more contemporary debates on instrumental, epistemic, and pragmatic rationality in science and psychology. The main argument is that rationality in psychology should be understood as a complex practice that integrates representation, belief assessment, values, goal choice, deliberation, and intervention. The entry concludes by proposing that rationality in psychology be conceived as a dynamic, contextual, and historically situated practice, as an approach that allows the discipline to be reconceived beyond the mere imitation of the natural sciences and opens space for a critical assessment of its practices, methods, interventions, and forms within the discipline.