Coenzyme Q is a lipid molecule widely diffused in nature; in humans and other mammals it is present as coenzyme Q 10. (CoQ 10). The first recognized role of CoQ 10 was in mitochondrial bioenergetics, where it plays a central role in the production of ATP. It is also present in other subcellular organelles, both in its oxidized and in its reduced state (ubiquinol-10). The reduced form of CoQ 10 is endowed with powerful antioxidant activity: it acts as a chain-breaking antioxidant and is also capable of regenerating alpha-tocopherol, the active form of vitamin E. By these mechanisms CoQ 10, together with vitamin E, protects lipoproteins from oxidation a process which bears considerable interest in preventing atherosclerosis. CoQ 10 has also been found to support cardiovascular function and the latest findings indicate an active role in counteracting endothelial dysfunction, which is closely implicated in cardiovascular disease. CoQ 10 also improves sperm motility, an effect which might be related both to its antioxidant and to its bioenergetic properties. Oxidative stress might be involved in neurodegenerative disease, and in migraine, two fields where the positive effects of CoQ 10 have been documented. CoQ 10 is synthesized by our body but is also present in food and can be taken as a nutritional supplement. The main source of industrially produced CoQ 10 is yeast fermentation. The process results in CoQ 10 which is identical to the naturally occurring molecule. Ubiquinol, the reduced form of CoQ 10, has recently become available.