A number of functions for coenzyme Q (CoQ) have been established during the years but its role as an effective antioxidant of the cellular membranes remains of dominating interest. This compound is our only endogenously synthesized lipid soluble antioxidant, present in all membranes and exceeding both in amount and efficiency that of other antioxidants. The protective effect is extended to lipids, proteins and DNA mainly because of its close localization to the oxidative events and the effective regeneration by continuous reduction at all locations. Its biosynthesis is influenced by nuclear receptors which may give the possibility, in the future, by using agonists or antagonists, of reestablishing the normal level in deficiencies caused by genetic mutations, aging or cardiomyopathy. An increase in CoQ concentration in specific cellular compartments in the presence of various types of oxidative stress appears to be of considerable interest.