Evolutionary medicine is a new field whose goal is to incorporate an evolutionary perspective into medical education, research, and practice. Evolutionary biologists and physicians have traditionally been concerned with different problems and have developed different ways of approaching and understanding biological phenomena. Evolutionary biologists analyze the properties of populations and the ways in which populations change over time, while physicians focus on the care of their individual patients. Evolutionists are concerned with the ultimate causes of biological phenomena, causes that operated during the phylogenetic history of a species, while physicians and biomedical scientists have been more interested in proximate causes, causes that operate during the ontogeny and life of an individual. Evolutionary medicine is based on the belief that an integration of these complementary views of biological phenomena will improve our understanding of health and disease. This essay reviews the theory of evolution by natural selection, as it was developed by Darwin and as it is now understood by evolutionary biologists. It emphasizes the importance of variation and selection, points out the differences between evolutionary fitness and health, and discusses some of the reasons why our evolutionary heritage has left us vulnerable to disease.