Free radical reactions are ubiquitous in living things. Studies on the origin and evolution of life provide a reasonable explanation for the prominent presence of this unruly class of chemical reactions. These reactions have been implicated in aging. Aging is the accumulation of changes responsible for the sequential alterations that accompany advancing age, and the associated progressive increases in the chance of disease and death. Aging changes are attributed to the environment and disease, and to an inborn process, the aging process. The latter produces aging changes at an exponentially increasing rate with advancing age. Improvements in general living conditions decrease the chance of death toward limiting values. Chances for death are now near these limits in the developed countries. Future significant increases in the average life span in the developed countries can only be achieved by slowing the rate of damage produced by the aging process. Support for the possibility that free radical reactions are responsible for the aging process now includes: i) studies on the origin of life and evolution; ii) studies on the effect of ionising radiation on living things; iii) dietary manipulations of endogenous free radical reactions; iv) the plausible explanations it provides for aging phenomena; and v) the growing numbers of studies that implicate free radical reactions in the pathogenesis of specific diseases. It is reasonable to expect on the basis of present data, that the average life expectancy at birth can be increased by 5 or more years by nutritious low caloric diets supplemented with one or more free radical reaction inhibitors.