Major new advances in the genetic and biochemical characterization of mitochondrial myopathies are discussed, within a general presentation of this important new area of human pathology. Mitochondrial disorders can be due to mutations in either nuclear or mitochondrial genes involved in the synthesis of individual respiratory chain subunits or in their posttranslational control. Although no mutations of nuclear-encoded oxidative phosphorylation subunits have been reported so far in humans, numerous biochemically defined disorders are attributed to nuclear gene defects. In contrast, molecular lesions of mitochondrial DNA are recognized as an increasingly frequent cause of defective oxidative phosphorylation. Numerous new mutations recently have been identified, including both maternally inherited point mutations and sporadic large-scale rearrangements. In addition, the identification of new or overlap syndromes has substantially broadened the clinical spectrum of mitochondrial disorders. To gain insight into the pathogenesis of these disorders, the relationship between specific clinical presentations and the mitochondrial genotype has been intensively investigated. In most cases, the phenotypic expression of the mitochondrial DNA mutations depends on the interplay among the relative amount of mutated vs wild-type genomes, ie, the degree of mitochondrial heteroplasmy and its tissue and cell distribution, the reliance of the affected tissues on aerobic energy supply, the age and gender of the individual, and other still poorly understood factors including individual "nuclear genetic background" and environmental factors.