Free radicals (FRs) are reactive chemical species that can damage or destroy biological molecules and have the potential for causing serious tissue and organ damage. Because of evidence suggesting that they are involved in the pathophysiology of a wide spectrum of human disorders, FRs have recently been the subject of considerable interest. While some popularized accounts depict FRs as always being destructive, essential physiological functions, including prostaglandin biosynthesis and phagoctye activity, depend upon them. It is only when FR reactions are uncontrolled that they are likely to be destructive. This article, based on citations from peer-reviewed biomedical literature, discusses some of the roles FRs play in essential physiological processes and their potential for inducing pathological changes. Also discussed are factors that add to body burdens of FRs and various antioxidant control mechanisms. The relationship of diet to FR pathology is emphasized because of evidence suggesting its importance in the prevention of chronic degenerative disorders including cardiovascular disease and cancer. The health benefits accruing from appropriate application of FR research are likely to be substantial. Accordingly, it is important for all health care practitioners to be aware of new developments in this area.