During the last decade, interest in assessing professionalism in medical education has increased exponentially and has led to the development of many new assessment tools. Efforts to validate the scores produced by tools designed to assess professionalism have lagged well behind the development of these tools. This paper provides a structured framework for collecting evidence to support the validity of assessments of professionalism. The paper begins with a short history of the concept of validity in the context of psychological assessment. It then describes Michael Kane's approach to validity as a structured argument. The majority of the paper then focuses on how Kane's framework can be applied to assessments of professionalism. Examples are provided from the literature, and recommendations for future investigation are made in areas where the literature is deficient.