Neuroendocrine (NE) tumors are a heterogeneous group of neoplasms arising in various organs and sharing the features of the NE cell system. The term "neuroendocrine" is used for cells characterized by their secretory products and some cytoplasmic proteins rather than by their localization and embryological derivation. Some tumor types can show a characteristic pattern on conventional histology but, to obtain an accurate diagnosis of many NE tumors it is necessary to employ various special methods, mainly electron microscopy and immuno-histochemistry. The classification of NE tumors in four categories according to Travis et al. is largely used: typical carcinoid and atypical carcinoid are low grade neoplasms; small cell NE carcinoma and large cell NE carcinoma are high grade neoplasms. The size, extension into surrounding tissues, angioinvasion and hormonal function are also important to consider in the prognostic evaluation of some NE tumors. Tumors exhibiting multidirectional differentiation must be classified in carcinoma with interspersed NE cells, carcinoid with interspersed non NE cells, composite tumors and amphricine tumors. Finally, some NE tumors may present features suggesting a Multiple Endocrine Neoplasia, ie. multifocality and association with hyperplasia of endocrine cells.