After a brief presentation of the data concerning the mechanism of production of oxidative stress as an expression of cellular and molecular stressing aggression, the main actions and functional implications of the excess of free radicals at the level of the main tissues and organs are presented. Emphasis is made on the nuclear, cytosolic and membrane alterations produced by the lipid peroxidation phenomena and the cell protein degradation induced by the oxidative stress. Special attention is given to the beneficial and harmful properties of the free radical species of oxygen and nitrogen, beginning with their involvement in the production of the normal adaptive reactions and ending with their involvement in the pathogenesis of various pathological states. In the group of beneficial effects are included along with the antibacterial and antiviral properties of the free radicals and their participation in the autoregulation of the local arteriolar-capillary circulation network. As harmful properties are mentioned the cellular destructive phenomena produced by the free radicals, which are considered as the generators of a true radicals pathology (senescence, atherogenesis, cancerogenesis, neurodegenerative diseases, etc).