The term "professionalism" has been used in a variety of ways. In 2012, the American Board of Medical Specialties (ABMS) Standing Committee on Ethics and Professionalism undertook to develop an operational definition of professionalism that would speak to the variety of certification and maintenance-of-certification activities undertaken by ABMS and its 24 member boards. In the course of this work, the authors reviewed prior definitions of professions and professionalism and found them to be largely descriptive, or built around lists of proposed professional attributes, values, and behaviors. The authors argue that while making lists of desirable professional characteristics is necessary and useful for teaching and assessment, it is not, by itself, sufficient either to fully define professionalism or to capture its social functions. Thus, the authors sought to extend earlier work by articulating a definition that explains professionalism as the motivating force for an occupational group to come together and create, publicly profess, and develop reliable mechanisms to enforce shared promises-all with the purpose of ensuring that practitioners are worthy of patients' and the public's trust.Using this framework, the authors argue that medical professionalism is a normative belief system about how best to organize and deliver health care. Believing in professionalism means accepting the premise that health professionals must come together to continually define, debate, declare, distribute, and enforce the shared competency standards and ethical values that govern their work. The authors identify three key implications of this new definition for individual clinicians and their professional organizations.