A free radical is an unstable and highly-reactive chemical species capable of independent existence that contained one or more unpaired electrons in its outer orbital. A number of oxygen-derived free radicals (ODFRs) have been identified. However, superoxide (O(-)(2) and hydroxyl (OH*) radicals are extensively studied. The univalent reduction of oxygen to water produces a number of highly-reactive chemical intermediates such as O(-)2 and OH*, which are commonly-known as oxygen-derived free radicals. ODFRS may be formed from several sources as follows: a) mitochondrial cytochrome oxidase, b) xanthine oxidase, c) neutrophils and d) transitional metals. There are several important defense mechanisms to limit or to prevent the damage caused by excessive ODFRs activity. These antioxidant defenses can be divided into a) enzymatic defense mechanisms such as: superoxide dismutase (SOD): catalase: selenium-containing glutathione peroxidase and b) non-enzymatic defense mechanisms including: alpha-tocopherol; ascorbic acid; glutathione and any sulfhydryl-containing compounds.