The article gives a review on oncocytic metaplasia and oncocytic tumors. Biochemical, cytochemical and molecular genetic findings are discussed. It has been established, that defects of the respiratory chain occur predominantly in oncocytic foci of the parathyroids and the liver. The underlying molecular genetic mechanisms are still unclear, however. In particular, one has to assume that oncocytic metaplasia differs pathogenetically from oncocytic neoplasia. Furthermore, organ-dependent prognostic differences of oncocytic tumors exist. Oncocytic carcinomas are predominantly seen in the thyroid and are scarce in other organs. The molecular genetic basis of this phenomenon is, however, still unknown.