Mitochondrial succinate dehydrogenase (SDH) consists merely of four nuclearly encoded subunits. It participates in the electron transfer in the respiratory chain and in succinate catabolism in the Krebs cycle. Mutations in the four genes, SDHA, B, C and D, have been reported, resulting in strikingly diverse clinical presentations. So far, SDHA mutations have been reported to cause an encephalomyopathy in childhood, while mutations in the genes encoding the other three subunits have been associated only with tumour formation. Following a brief description of SDH genes and subunits, we examine the properties and roles of SDH in the mitochondria. This allows further discussion of the several hypotheses proposed to account for the different clinical presentations resulting from impaired activity of the enzyme. Finally we stress the importance of SDH as a target and/or marker in a number of diseases and the need to better delineate the consequences of SDH deficiency in humans.