Coenzyme Q10 (CoQ10) is a lipophilic component of the mitochondrial respiratory enzyme chain, which transfers electrons to complex III (cytochrome bc1 complex) from complex I (NADH-CoQ reductase), complex II (succinate dehydrogenase), and from the oxidation of fatty acids and branched-chain amino acids via flavin-linked dehydrogenases. Additional cellular functions of CoQ10 have been described. Deficiency of CoQ10 in muscle has been identified in patients with a spectrum of encephalomyopathies ranging from predominant cerebellar ataxia to pure myopathy. In a family with two children affected by infantile-onset encephalomyopathy and nephropathy, the first mutation in a CoQ biosynthetic gene, COQ2, was identified, thus proving the existence of primary CoQ10 deficiency. This article reviews the current state-of-knowledge regarding biochemical and molecular genetic features of inherited CoQ10 deficiency.