Medical education practice is more often the result of tradition, ritual, culture, and history than of any easily expressed theoretical or conceptual framework. The authors explain the importance and nature of the role of theory in the design and conduct of graduate medical education. They outline three groups of theories relevant to graduate medical education: bioscience theories, learning theories, and sociocultural theories. Bioscience theories are familiar to many medical educators but are often misperceived as truths rather than theories. Theories from such disciplines as neuroscience, kinesiology, and cognitive psychology offer insights into areas such as memory formation, motor skills acquisition, diagnostic decision making, and instructional design. Learning theories, primarily emerging from psychology and education, are also popular within medical education. Although widely employed, not all learning theories have robust evidence bases. Nonetheless, many important notions within medical education are derived from learning theories, including self-monitoring, legitimate peripheral participation, and simulation design enabling sustained deliberate practice. Sociocultural theories, which are common in the wider education literature but have been largely overlooked within medical education, are inherently concerned with contexts and systems and provide lenses that selectively highlight different aspects of medical education. They challenge educators to reconceptualize the goals of medical education, to illuminate maladaptive processes, and to untangle problems such as career choice, interprofessional communication, and the hidden curriculum.Theories make visible existing problems and enable educators to ask new and important questions. The authors encourage medical educators to gain greater understanding of theories that guide their educational practices.