This narrative review, published in Medical Education, summarizes articles reporting on burnout among medical students and residents (trainees) published between 1990 and 2015. Studies suggest a high prevalence of burnout among trainees, with levels higher than in the general population. Burnout can undermine trainees’ professional development, place patients at risk, and contribute to a variety of personal consequences, including suicidal ideation. Factors within the learning and work environment, rather than individual attributes, are the major drivers of burnout. Limited data are available regarding how to best address trainee burnout, but multi-pronged efforts, with attention to culture, the learning and work environment and individual behaviors, are needed to promote trainees’ wellness and to help those in distress.