Mitochondrial disorders are clinically, biochemically, and genetically heterogeneous and surprisingly diverse. The clinical spectrum ranges from severe multisystem disorders of early infancy to monosymptomatic mild disorders in late adulthood. Causative for mitochondrial dysfunction are mutations in nuclear genes or/and the proper mitochondrial genome, the mitochondrial DNA. The last years have seen quite remarkable progress in identification of the molecular basis of mitochondrial diseases, which helps in the often difficult process of diagnosis. However, the clinical variability and complexity continue to increase and remain unexplained in many cases.