The nature of the primary aging processes has yet to be defined. Mortality and aspects of longevity (eg, life expectancy) do not provide significant information about aging. The maximum life span potential of a species does provide information about the comparative rate of aging between species, however, and also is a good marker for evaluating interventions aimed at modulating the rate of aging in a particular species. Physiological deterioration occurs with age, but it is not known how much is due to aging per se and how much is secondary to age-related disease and life-style. There are similar reservations about the changes in body composition that have been noted with increasing age. Many diseases are age related in terms of prevalence, but how these diseases relate to the primary aging processes remains to be defined. Finally, many claims have been made concerning interventions that will retard the aging processes; of these, only food restriction in rodents has been unequivocally shown to retard the aging processes of a mammalian species.