Biological aging is associated with increased cellular levels of reactive oxygen species (ROS) as well as the formation and accumulation of oxidized biomolecules. During evolution, organisms developed a highly-efficient and adaptive antioxidant defense system. Antioxidants can generally be divided into two categories: enzymatic and non-enzymatic. During the aging process the activity of antioxidant enzymes, e.g. SOD, CAT, GSH-Px, and GSSG-R, depends on factors such as race, gender, tissue and subcellular localization of enzymes. The age-dependent decrease in antioxidant enzyme activity may be attributed to oxidative modifications of enzymes. During the aging process, ROS may also lead to the induction of some enzyme activity which is explained as an adaptive phenomenon. The decrease in GSH concentration with age can be explained by decreased GSH synthesis and/or increased GSH consumption in the removal of peroxides and xenobiotics. In plasma albumin, ferritin, transferrin, and caeruloplasmin exert protective action. Plasma proteins can inhibit ROS generation and lipid peroxidation by chelating free transition metals. Plasma protein concentrations changes with age. The major exogenous antioxidants, mostly derived from the diet, are vitamin E, C, A, and beta-carotene. During the aging process the level of vitamins may decrease or increase, depending on such factors as diet, and diseases.