The 16569 base pairs of the mitochondrial DNA encode with a specific genetic code 13 proteins involved in the respiratory chain complex formation. Nuclear gene products also contribute to the formation of these complexes. In the first point, the organization and expression of the mtDNA are described with the main characteristics of the enzymatic complexes as well as nuclear gene expression. New information concerned with mitochondrial DNA deletions and mutations are described particularly with respect to Kearns-Sayre Syndrome.