The study of free radical processes and elucidation of their roles in the functioning of cells under normal conditions and during the development of different pathological states are serious problems of modern biology and medicine. In her monograph E.E. Dubinina has presented the detailed characteristics of reactive oxygen species and other free radicals, information on the main pathways of their generation in the body, and enzymatic and non-enzymatic systems of antioxidant protection (Chapter 1) on the basis of an analysis of the literature of the last 10–15 years.