Fitness to practice decisions are often based on a student's digression from the regulations, with limited exploration of the reasoning behind the student's behavior. However, behavior is underpinned by complex, "hidden" variables, including an individual's attitudes and social norms. Examining hidden determinants of professionalism, such as context, interpersonal relationships, social norms, and local cultures, then allows medical educators to develop a richer understanding of unprofessional behavior.In this article, the authors propose the use of the theory of planned behavior (TPB) as a framework to help evaluate unprofessional behavior in students. The TPB is a deliberative processing model that explains how an individual's behavior is underpinned by his or her cognitions, with behavior being primarily dependent on the intention to perform the behavior (behavioral intention). Intention, in turn, is determined by three variables: attitude, subjective norm, and perceived behavioral control.To understand the practical use of the TPB, the authors present four complex, anonymized case studies in which they employed the TPB to help deal with serious professionalism lapses among medical students. The outcomes of these cases as well as the student and program director perspectives, all explained via the TPB variables, are presented. The strengths and limitations of the TPB are discussed.