Aging is the progressive accumulation of changes with time associated with and/or responsible for the ever-increasing susceptibility to disease and death. These time-related changes are attributed to the aging process. This process may be due in part to free radical reactions, largely those involving oxygen, going on continuously throughout the cells and tissues. Dietary manipulations expected to lower the rate of production of free-radical reaction damage have been shown to 1) increase the life span of mice, rats, fruit flies, nematodes, and rotifers, as well as the life span of neurospora, 2) inhibit development of some forms of cancer, 3) enhance humoral and cell-mediated immune responses, and 4) slow development of amyloidosis and the autoimmune disorders of NZB mice. Free-radical reactions may also play a significant role in the deterioration of the cardiovascular and central nervous systems with age. It is reasonable to expect, on the basis of present data, that the healthy life span can be increased by five 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.