For a number of years, coenzyme Q 10 (CoQ 10 ) was known for its key role in mitochondrial bioenergetics; later studies demonstrated its presence in other subcellular fractions and in blood plasma, and extensively investigated its antioxidant role. These 2 functions constitute the basis for supporting the clinical use of CoQ 10 . Also, at the inner mitochondrial membrane level, CoQ 10 is recognized as an obligatory cofactor for the function of uncoupling proteins and a modulator of the mitochondrial transition pore. Furthermore, recent data indicate that CoQ 10 affects the expression of genes involved in human cell signaling, metabolism and transport, and some of the effects of CoQ 10 supplementation may be due to this property. CoQ 10 deficiencies are due to autosomal recessive mutations, mitochondrial diseases, aging-related oxidative stress and carcinogenesis processes, and also statin treatment. Many neurodegenerative disorders, diabetes, cancer, and muscular and cardiovascular diseases have been associated with low CoQ 10 levels as well as different ataxias and encephalomyopathies. CoQ 10 treatment does not cause serious adverse effects in humans and new formuhave been developed that increase CoQ 10 absorption and tissue distribution. Oral administration of CoQ 10 is a frequent antioxidant strategy in many diseases that may provide a significant symptomatic benefit.