Darwinian medicine applies principles of evolutionary adaptation and natural selection to the study of disease, emphasizing that a full understanding of medical problems requires an understanding of why states of disease exist. There are three evolutionary explanations for the presence of disease: natural selection is insufficiently strong to eliminate the disease, natural selection has had insufficient time to eliminate the causes of disease, and conflicting actions of natural selection maintain disease. This entry illustrates these categories with examples that are widely cited or well-founded and assesses the extent to which they explain significant health problems. This assessment emphasizes that the complete spectrum of alternative explanations needs to be considered to understand the causes of disease and improve efforts to prevent and treat disease.