Cell metabolism in organisms which use oxygen as a source of energy is closely associated with the generation and action of free oxygen radicals and their derivatives. Extra- and intracellular substances that are antioxidative in nature prevent overproduction of radicals and protect against propagation of peroxidative reactions. The list of compounds which can be treated as antioxidants becomes elongated. Many classifications of these compounds are used, of which the most common is the division according to their nature into enzymatic and non-enzymatic, according to their environment or the way they react with FOR. Enzymatic antioxidants include: superoxide dismutase, catalase, glutathione peroxidase and glutathione reductase. Non-enzymatic antioxidants are: vitamin E, vitamin C, glutathione, carotenes and retinols, and some transition metals (Zn, Cu and Se). The balance between the actions of these two groups of compounds determines normal functioning of the organism. Impairment of the balance between pro- and antioxidative processes in the organism is called anitoxidative stress and may be induced by intensified reactions involving FOR and by depressed activity/concentration of antioxidants. It seems, however, that irrespective of the cause, oxidative stress is likely to result in many diseases.