The term "free radical" refers to atoms or groups of atoms containing a single unpaired electron occupying an outer orbital. This unpaired electron lends the molecule distinct chemical and physical properties such as its high reactivity and magnetic moment, respectively. Some relevant advances in medical research have encompassed O2 radicals and other toxic derivatives of O2. These reactive O2 species were shown to play a significant role in a wide spectrum of pathological states (see Halliwell & Gutteridge, 1985; Flohé 1986; Kensler & Taffe, 1986; McCord, 1986), though the measurement of free radical activity in clinically-accesible material has been unsatisfactory (Dormandy, 1986). This overview provides only a framework of information to assist the reader on some relevant topics on free radical pathology, which by no means can cover the multiple aspects of the medical literature in this context. The reader is referred to several articles which cover particular features of free radical biology and medicine (Chance et al., 1979; Pryor, 1976-1984; Fridovich, 1983; Jamieson et al., 1986; Kensler & Taffe, 1986; McCord, 1986; Weiss, 1986).