Empathy is important to patient care. It enhances patients' satisfaction, comfort, self-efficacy, and trust which in turn may facilitate better diagnosis, shared decision making, and therapy adherence. Empathetic doctors experience greater job satisfaction and psychological well-being. Understanding the development of empathy of tomorrow's health care professionals is important. However, clinical empathy is poorly defined and difficult to measure, while ways to enhance it remain unclear. This review examines empathy among undergraduate medical students, focusing upon three main questions: How is empathy measured? This section discusses the problems of assessing empathy and outlines the utility of the Jefferson Scale of Empathy - Student Version and Davis's Interpersonal Reactivity Index. Both have been used widely to assess medical students' empathy. Does empathy change during undergraduate medical education? The trajectory of empathy during undergraduate medical education has been and continues to be debated. Potential reasons for contrasting results of studies are outlined. What factors may influence the development of empathy? Although the influence of sex is widely recognized, the impact of culture, psychological well-being, and aspects of undergraduate curricula are less well understood. This review identifies three interrelated issues for future research into undergraduate medical students' empathy. First, the need for greater clarity of definition, recognizing that empathy is multidimensional. Second, the need to develop meaningful ways of measuring empathy which include its component dimensions and which are relevant to patients' experiences. Medical education research has generally relied upon single, self-report instruments, which have utility across large populations but are limited. Finally, there is a need for greater methodological rigor in investigating the possible determinants of clinical empathy in medical education. Greater specificity of context and the incorporation of work from other disciplines may facilitate this.