The results of histopathological, histochemical and ultrastructural investigations on pheochromocytomas and paragangliomas have been reported. These results allowed the functional identification of the cell types composing many of such tumours. Moreover, comparison of these data with clinico-pathologic findings outlined the advantages and limits of cytologic studies for understanding the natural history of pheochromocytomas and paragangliomas and improving our diagnostic and prognostic criteria.