The role of free radicals and lipid peroxidation is reviewed with regard to the aging process. Free radicals are produced during mitochondrial respiration, during the autooxidation of a variety of biological molecules and chemicals, during irradiation damage, and are found as environmental pollutants. Free radicals induce lipid peroxidation which results in membrane damage, increased disulfide/sulfhydryl ratios, and accumulation of aging pigments. Superoxide dismutase, glutathione peroxidase, vitamin E, vitamin C, and selenium are of importance with respect to free radical and lipid peroxide quenching. During aging, the levels of vitamin C appear to decline in the human, guinea pig, and the mouse. Synthetic antioxidants, added to the diets of mice, have been noted to extend the lifespan and mean half-survivale times.