The interrelationships between diet and life span are reviewed, with emphasis on results obtained in studies with experimental animals. Ad libitum feeding throughout life does not promote maximal survival and food restriction increases mean life span. The possible importance of total energy, carbohydrate, and protein intake are considered, as well as the influence of mode of feeding. A number of mechanisms that might explain how nutrition affects life span are discussed, including disease pattern, free radicals, neuroendocrine system, and protein turnover. The significance of these findings in relation to the health and life span of human subjects is not known and deserves further exploration. However, they emphasize the importance of diet as a tool to help explore the biochemical and physiological basis for aging in animal models.