Free oxygen radicals are a highly reactive species of molecules characterized by an unpaired electron in the outer electron ring. They are formed continuously in the organism as part of enzymatic reactions, or as byproducts of oxidation processes. Antioxidants are able to eliminate free oxygen radicals, but during ischaemic reperfusion, the capacity of these systems is exceeded resulting in tissue injury, cell necrosis, destruction of enzymes and DNA-damage. Free oxygen radicals thereby play a major role in the tissue injury seen in organs after transplantation surgery and aorta operations. When the endogenous antioxidative capacity is exceeded, protection of lipid membranes against attack by free oxygen radicals by antioxidant supplementation is an alternative strategy for reduction of tissue injury. Since antioxidants are atoxic, they could also be of importance even in small operations, where the tissue blood supply is often periodically cut off.