Validity arguments for assessment tools intended to measure medical professionalism suffer for lack of a clear theoretical statement of what professionalism is and how it should behave. Drawing on several decades of field research addressing deviance and informal social control among physicians, a theoretical sketch of professionalism is presented that can be used to predict how individual adjustment to professional norms should co-vary with other social and psychological phenomena. Physicians may understand and value professional norms but fail to enact them in practice because of conflicting normative demands. Physicians' social networks are predicted to act as conduits of social learning and social pressure, driving the resolution of normative conflicts in specific directions. A valid assessment of professionalism requires an adequate accounting of the social reaction to an individual's professional conduct, because the reaction and the conduct itself are inseparable.