The aim of the present work has been to provide insight into a new, rapidly developing field of medicine. Although the title may suggest a purely theoretical approach, our purpose has been to highlight the important practical aspects of the subject. Several detailed and comprehensive studies are available dealing with the chemistry and biology of free radical reactions; we, therefore, only wish to review the medical implications. It may seem premature to draw practical conclusions in a field where some of the basic biological and biochemical mechanisms have not yet been adequately clarified, still we feel that the potentialities involved justify our calling attention to them. This work is chiefly meant for doctors and medical students; therefore, after a chemical introduction, the role of free radical reactions in physiological processes and human diseases, and the relevant therapeutic possibilities will be discussed.