Knowledge of laughter, other than of its clinical manifestations, is tenuous. A unique, ubiquitous human phenomenon, laughter has largely been neglected by medical investigators and relegated to philosophers and naturalists, particularly Darwin. Clinical manifestations are well described, but the many physiological changes accompanying laughter are not. Neurological pathways remain speculative, and those described are based on pathological evidence. Laughter, as a behavior, has many causes, including humor, incongruity, relief, and a sense of well-being. Laughter may have developed for its cathartic effects and adaptive value. As an evolutionary device, laughter may have served to thwart aggression, spread information, and preserve social unity. Laughter's potential for a variety of clinical uses awaits future investigation.