Aging is the progressive accumulation of changes with time that are associated with or responsible for the ever-increasing susceptibility to disease and death which accompanies advancing age. These time-related changes are attributed to the aging process. The nature of the aging process has been the subject of considerable speculation. Accumulating evidence now indicates that the sum of the deleterious free radical reactions going on continuously throughout the cells and tissues constitutes the aging process or is a major contributor to it. In mammalian systems the free radical reactions are largely those involving oxygen. Dietary manipulations expected to lower the rate of production of free radical reaction damage have been shown (i) to increase the life span of mice, rats, fruit flies, nematodes, and rotifers, as well as the "life span" of neurospora; (ii) to inhibit development of some forms of cancer; (iii) to enhance humoral and cell-mediated immune responses; and (iv) to slow development of amyloidosis and the autoimmune disorders of NZB and NZB/NZW mice. In addition, studies strongly suggest that free radical reactions play a significant role in the deterioration of the cardiovascular and central nervous systems with age. The free radical theory of aging provides reasonable explanations for age-associated phenomena, including (i) the relationship of the average life spans of mammalian species to their basal metabolic rates, (ii) the clustering of degenerative diseases in the terminal part of the life span, (iii) the beneficial effect of food restriction on life span, (iv) the greater longevity of females, and (v) the increase in autoimmune manifestations with age. It is not unreasonable to expect on the basis of present data that the healthy life span can be increased by 5-10 or more years by keeping body weight down, at a level compatible with a sense of well-being, while ingesting diets adequate in essential nutrients but designed to minimize random free radical reactions in the body.