It is commonly believed that diet can influence aging processes, but the evidence in support of such an action in humans is weak. The criteria for evaluating the effect of diet on aging processes are discussed. Using these criteria, only one dietary manipulation has been clearly shown to influence the aging processes in a mammalian species, and that is the reduction of dietary energy intake by laboratory rodents. Possible mechanisms by which the reduction of energy intake slows the aging processes in rodents are discussed.