This AMEE guide provides a research overview of the identification of, and responding to unprofessional behaviour in medical students. It is directed towards medical educators in preclinical and clinical undergraduate medical education. It aims to describe, clarify and categorize different types of unprofessional behaviours, highlighting students' unprofessional behaviour profiles and what they mean for further guidance. This facilitates identification, addressing, reporting and remediation of different types of unprofessional behaviour in different types of students in undergraduate medical education. Professionalism, professional behaviour and professional identity formation are three different viewpoints in medical education and research. Teaching and assessing professionalism, promoting professional identity formation, is the positive approach. An inevitable consequence is that teachers sometimes are confronted with unprofessional behaviour. When this happens, a complementary approach is needed. How to effectively respond to unprofessional behaviour deserves our attention, owing to the amount of time, effort and resources spent by teachers in managing unprofessional behaviour of medical students. Clinical and medical educators find it hard to address unprofessional behaviour and turn toward refraining from handling it, thus leading to the 'failure to fail' phenomenon. Finding the ways to describe and categorize observed unprofessional behaviour of students encourages teachers to take the appropriate actions. . She received her MD in 1986, and completed residency training in general practice at the University of Amsterdam in 1990. Marianne worked in general practice for 15 years, combining this work with teaching clerks in the workplace, and from 2001 with other educational activities at the Faculty of Medicine at the VU University. From 2010 onwards, she has been coordinating the educational theme 'Professional behaviour' at the school. Recently, Marianne received her PhD with distinction. Her research explores how educators and peer students perceive and respond to unprofessional behaviours of medical students.Arianne Teherani PhD, is a Professor of role in which she leads design, development, and policy for evaluation of the school's curriculum and programs. Arianne's main research interests are in the areas of professionalism, professional identity formation, and equity in medical education. She has led studies aimed at identification and remediation of unprofessional behaviours, the role of clinical education in shaping the professional identity of learners and teachers, and the role of assessment and learning environment practices in perpetuating educational disparities and interventions aimed at creating equity during medical school. . His current medical education research focuses on generic aspects of (post)graduate training, and on intensive care medicine specifically, including the supervision of national and international PhD students. In addition he performs research in the fields of organ donatio...