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.