This brief survey of laughter attempts an analysis of its neurological mechanisms, evolution, role in social behaviour and its clinicopathological importance. The mechanisms of laughter, its physiological consequences and its demonstration by sound spectrography are considered. Something resembling laughter occurs in certain primates, and possibly rodents, though there are important differences. The evolution of laughter in a social context is appraised. Pathological laughter arises rarely, usually caused by diseases of the frontal or temporal lobes, and in hypothalamic hamartomata in children.