Living in aerobic conditions carries a risk of oxidative stress, in connection with free radical deleterious action on tissues and cells. Free radical mechanisms have been implicated in the pathogenesis of many diseases, as well as in host defense against various invading microorganisms. A substantial body of evidence has been reported on free radical involvement in inflammation, ischaemia/reperfusion injury, atherosclerosis and many other pathologies. The aim of this paper is to review selected literature and opinions concerning free radical-induced damage to tissues and to present xenobiotic contribution to oxidative stress.