Known endogenous antioxidants are unlikely to prevent radical damage due to oxidative stress or achieve complete repair by established reaction mechanisms. While near complete prevention seems very unrealistic, some of the initial damage can be repaired. Depending on tissue, this may be even a large fraction. Antioxidants, however, will efficiently break radical reaction chains and, therefore, certainly limit the damage caused by radicals. It is not clear if chemical antioxidant action is strictly limited to electron-transfer processes or if additional reaction mechanisms may contribute.