Originating from cells of the diffuse endocrine system the endocrine tumours of the gut and the pancreatic tract are rare entities characterized by a common phenotypic aspect and producing several bioactive substances including growth factors. Two major categories are identified: well-differentiated and poorly differentiated tumours. The clinical behaviour varies ranging from benign to low grade malignant for well-differentiated tumours/carcinomas to high grade malignant for poorly differentiated carcinomas. The two major categories of well-differentiated and poorly differentiated tumours display distinct phenotypes and genetic backgrounds possibly supporting distinct histogenesis. Genetic abnormalities associated with either induction or progression of tumours may vary depending on the site of origin.