The inclusion of the humanities in medical education may offer significant potential benefits to individual future physicians and to the medical community as a whole. Debate remains, however, about the definition and precise role of the humanities in medical education, whether at the premedical, medical school, or postgraduate level. Recent trends have revealed an increasing presence of the humanities in medical training. This article reviews the literature on the impact of humanities education on the performance of medical students and residents and the challenges posed by the evaluation of the impact of humanities in medical education. Students who major in the humanities as college students perform just as well, if not better, than their peers with science backgrounds during medical school and in residency on objective measures of achievement such as National Board of Medical Examiners scores and academic grades. Although many humanities electives and courses are offered in premedical and medical school curricula, measuring and quantifying their impact has proven challenging because the courses are diverse in content and goals. Many of the published studies involve self-selected groups of students and seek to measure subjective outcomes which are difficult to measure, such as increases in empathy, professionalism, and self-care. Further research is needed to define the optimal role for humanities education in medical training; in particular, more quantitative studies are needed to examine the impact that it may have on physician performance beyond medical school and residency. Medical educators must consider what potential benefits humanities education can contribute to medical education, how its impact can be measured, and what ultimate outcomes we hope to achieve.