This article is a brief review of current knowledge concerning some basic concepts about the redox and addition chemistry of quinones, followd by a survey of current information regarding the biochemistry of quinones in mammalian cells and their pro- and antioxidant functions. In recent years it has been recognized that ubiquinone (Coenzyme Q), in addition to its involvement as an electron and proton carrier in mitochondrial and bacterial respiration, acts in its reduced form (ubiquinol) as an antioxidant. The antioxidant activity, together with its high degree of hydrophobicity and its widespread occurrence in biological membranes and in low-density lipoprotein, suggest un important role of ubiquinol in cellular defense against oxidative damage. Degenerative diseases and aging may be manifestations of a decreased capacity to maintain adequate ubiquinol levels.