Oxygen in absolutely necessary to life but it is also a toxic gas. 1 to 2% of molecular oxygen undergoes an univalent reduction which produces very reactive and very cytotoxic species. Against them there are different protector antioxidant systems, called scavengers. Pathologically four points are fundamental: Free radicals have a main role in inflammation and fight against bacteria. In carcinogenesis, they have a key role in promotion. The cellular ageing appears to be imputable to a defect of the scavengers. Reflow following ischemia involves toxic free radicals. To prevent the tissue injury due to reperfusion pre treatment by SOD, catalase, allopurinol are at their beginning but the first results are hopeful for skin, kidney, heart and pancreas.