In recent years, professionalism in medicine has gained increasing attention. Many have called for a return to medical professionalism as a way to respond to the corporate transformation of the U.S. health care system. Yet there is no common understanding of what is meant by the word professionalism. To encourage dialog and to arrive eventually at some consensus, one needs a normative definition. The author proposes such a definition and asserts that the concept of medical professionalism must be grounded both in the nature of a profession and in the nature of physicians' work. Attributes of medical professionalism reflect societal expectations as they relate to physicians' responsibilities, not only to individual patients but to wider communities as well. The author identifies nine behaviors that constitute medical professionalism and that physicians must exhibit if they are to meet their obligations to their patients, their communities, and their profession. (For example, “Physicians subordinate their own interests to the interests of others.”) He argues that physicians must fully comprehend what medical professionalism entails. Serious negative consequences will ensue if physicians cease to exemplify the behaviors that constitute medical professionalism and hence abrogate their responsibilities both to their patients and to their chosen calling.