Toxic oxygen free radicals have been implicated as important pathologic mediators in many clinical disorders. We discuss the chemistry of oxygen radical production and the roles of iron and of various antioxidants as well as the diseases that have received active attention in oxy-radical research. Particular attention is focused on cigarette smoke oxidants, ischemia-reperfusion-induced radical production, carcinogenesis, and aging. Such research may well provide a firm foundation for therapeutic breakthroughs.