The importance of food restriction in rats, mice, and hamsters as a tool for the study of aging is discussed. The evidence that food restriction retards the aging processes is summarized and includes its ability to extend the maximum life span, to decrease the rate of increase in age-specific mortality, to retard age changes in physiological processes, and to delay or prevent most age-associated diseases. Food restriction has its anti-aging action by reducing the intake of energy rather than a specific nutrient. Research findings on the possible mechanisms by which food restriction retards the aging processes are discussed. The use of food restriction to test theories of aging is described and the controversial issue of its possible use as an intervention of human aging is addressed.