This chapter points out medical professionalism that should include maintaining up-to-date knowledge as an important component. It argues that expertise and professionalism are inextricably linked. Medical professionalism is demonstrated by what the physicians do with patients and their communities. The relationship between the medical profession and society is changing owing to a number of forces in and outside medicine. Medical knowledge and practice skills are rapidly increasing with new scientific discoveries in genetics, bioengineering, informatics and technology. The chapter reviews how this issue has been reflected in the various components of medical training, accreditation and practice in the United States. The ethics of the commercialism that entered into healthcare in many countries, especially from the pharmaceutical industry, is often counter to the ethics of medical professionalism. Physicians have thus been challenged to preserve the doctor-patient relationship and to withstand threats to their professionalism.