Free radicals and other oxygen-derived species are constantly generated in vivo, both by “accidents of chemistry” and for specific metabolic purposes. The reactivity of different free radicals varies, but some can cause severe damage to biological molecules, especially to DNA, lipids, and proteins. Antioxidant defense systems scavenge and minimize the formation of oxygen-derived species, but they are not 100% effective. Hence, diet-derived antioxidants may be particularly important in diminishing cumulative oxidative damage and helping us to stay healthier for longer. Repair systems exist to deal with molecules that have been oxidatively damaged. Damage to DNA by hydroxyl radicals appears to occur in all aerobic cells, and might be a significant contributor to the age-dependent development of cancer. Lipid peroxidation probably contributes significantly to the development of atherosclerosis.