In this paper we review the diseases, such as cancer, arteriosclerosis, arthritis, neurodegenerative disorders, whose emerging pathogenetic evidence by Free Radicals has been shown either from experimental or from clinical point of view. In the last 10 years the growing evidence of a co-causative responsibility of oxidative stress in some chronic and acute illnesses highlighted the need to improve the diagnostic potential as well as the algorythm of an effective treatment plan. An exhaustive basic description of the Free Radicals action mechanism in different parenchymas to produce damage and clinical symptoms is the very preliminary background to any further investigation as to the specific diseases-oriented diagnostic tests as well as to a rationale therapy.