Professionalism is a core competency of physicians. In this article, the statements of professional societies (e.g., the Charter on Medical Professionalism), the expectations of patients and society regarding professionalism, and a framework for defining medical professionalism are described. The framework's foundation consists of clinical competence, communication skills, and a sound understanding of the ethical and legal aspects of medicine. Rising from this foundation are attributes of professionalism: accountability, altruism, excellence, and humanism. The capstone of the framework is professionalism, or the complete physician. Reasons for teaching professionalism to and assessing professionalism among medical students, physicians in training, and physicians in practice are also described. These reasons include patient expectations; the association between professionalism and improved clinical outcomes (and the association between unprofessional behavior and adverse outcomes); accreditation organization requirements; and observations that professionalism can be taught, learned, and assessed. In addition, methods for teaching professionalism are described (e.g., didactic lectures, discussion groups, simulation, and role-modeling). To ensure that medical students, physicians in training, and physicians in practice are competent in professionalism, they should be assessed for professionalism. Thus, approaches to assessing professionalism are also described (e.g., multiple tools and observers). Professionalism assessments can be used for formative and summative feedback, evaluation of professionalism education programs, and generating hypotheses for professionalism research. Finally, the rich history and culture of clinical excellence and professionalism and specific programs for teaching and assessing professionalism at Mayo Clinic are described throughout this article. Indeed, the Mayo Clinic experience validates professionalism as a core physician competency.