Background: Developing professionalism is a core task in medical education. Unfortunately, it has remained difficult for educators to identify medical students' unprofessionalism, because, among other reasons, there are no commonly adopted descriptors that can be used to document students' unprofessional behaviour. This study aimed to generate an overview of descriptors for unprofessional behaviour based on research evidence of real-life unprofessional behaviours of medical students. Methods: A systematic review was conducted searching PubMed, Ebsco/ERIC, Ebsco/PsycINFO and Embase.com from inception to 2016. Articles were reviewed for admitted or witnessed unprofessional behaviours of undergraduate medical students. Results: The search yielded 11,963 different studies, 46 met all inclusion criteria. We found 205 different descriptions of unprofessional behaviours, which were coded into 30 different descriptors, and subsequently classified in four behavioural themes: failure to engage, dishonest behaviour, disrespectful behaviour, and poor self-awareness. Conclusions: This overview provides a common language to describe medical students' unprofessional behaviour. The framework of descriptors is proposed as a tool for educators to denominate students' unprofessional behaviours. The found behaviours can have various causes, which should be explored in a discussion with the student about personal, interpersonal and/or institutional circumstances in which the behaviour occurred. Explicitly denominating unprofessional behaviour serves two goals: [i] creating a culture in which unprofessional behaviour is acknowledged, [ii] targeting students who need extra guidance. Both are important to avoid unprofessional behaviour among future doctors.