The tumours of the disseminated/diffuse neuroendocrine cell system are a group of neoplasms sharing uniformly appearing cells which differ from each other in their biology, prognosis and genetics. In the lung they are called carcinoid and small/large-cell neuroendocrine carcinomas. In the gastroenteropancreatic compartment they are classified as well-differentiated neuroendocrine tumours or carcinomas and poorly differentiated neuroendocrine carcinomas. Depending on their localization these neoplasms reveal distinct phenotypes with respect to pathology, immunohistochemistry, and hormonal syndromes. Their clinical behaviour--ranging from benign and low-grade to high-grade malignancy--can be predicted on the basis of clinicopathological criteria. Currently extensive work is being performed to unravel the genetic background.