The first description of a patient with mitochondrial myopathy and deficient respiratory chain function was reported by Luft and coworkers almost 40 years ago. Subsequent studies in the 1970s and 1980s relied on a combination of morphological and biochemical methods to identify patients with mitochondrial disorders. However, the aetiology and pathogenesis remained largely unsolved and there was poor correlation between laboratory findings and clinical phenotypes. The fact that both mitochondrial DNA (mtDNA) and nuclear genes are necessary for the biogenesis of the respiratory chain, suggested that mutations of either genome might cause mitochondrial myopathy. This prediction has been verified during the last decade and pathogenic mutations of both genomes have been identified. This rapid accumulation of genetic information has lead to many good correlations between genotype and phenotype in mitochondrial disorders. The challenge for the future will be to elucidate molecular details of pathogenic processes and to develop effective treatments for patients with respiratory chain dysfunction.