This presentation is a brief review of current knowledge concerning some biochemical, physiological and medical aspects of the function of ubiquinone (coenzyme Q) in mammalian organisms. In addition to its well-established function as a component of the mitochondrial respiratory chain, ubiquinone has in recent years acquired increasing attention with regard to its function in the reduced form (ubiquinol) as an antioxidant. Ubiquinone, partly in the reduced form, occurs in all cellular membranes as well as in blood serum and in serum lipoproteins. Ubiquinol efficiently protects membrane phospholipids and serum low-density lipoprotein from lipid peroxidation, and, as recent data indicate, also mitochondrial membrane proteins and DNA from free-radical induced oxidative damage. These effects of ubiquinol are independent of those of exogenous antioxidants, such as vitamin E, although ubiquinol can also potentiate the effect of vitamin E by regenerating it from its oxidized form. Tissue ubiquinone levels are regulated through the mevalonate pathway, increasing upon various forms of oxidative stress, and decreasing during aging. Drugs inhibiting cholesterol biosynthesis via the mevalonate pathway may inhibit or stimulate ubiquinone biosynthesis, depending on their site of action. Administration of ubiquinone as a dietary supplement seems to lead primarily to increased serum levels, which may account for most of the reported beneficial effects of ubiquinone intake in various instances of experimental and clinical medicine.