This chapter defines professionalism as a construct of attitudes and beliefs that will determine whether an individual will engage in professional or unprofessional behaviour. This approach to professionalism is now being advocated by researchers in the field as a means to facilitate understanding and training in professionalism. The chapter discusses the four main issues: the need for formal training in professionalism; the evidence for effectiveness of training in professionalism; the role of assessment in driving learning to become a professional; and the theoretical underpinning of interventions designed to facilitate professionalism in medicine. Professionalism-related issues form the basis for the majority of complaints against doctors. Regulating bodies such as the General Medical Council (GMC) and the American Board of Internal Medicine (ABIM) now explicitly state that training to become a professional must include not just acquisition of professional knowledge and skills, but also of appropriate attitudes and behaviours.