It has been said that with the description of the Zollinger-Ellison syndrome in 1955, the clinical era of gastrointestinal endocrinology was inaugurated. Since that time, a virtual explosion of investigations, both basic and clinical, has occurred in which as many as 19 gastroenteropancreatic neuroendocrine cells have been identified and as many as 40 of their humoral products have been discovered. The pharmacologic and physiologic functions of some of these amines and peptides have been clearly identified and account for the clinical presentations of the various clinical syndromes so far described. However, the physiologic functions of many others remain to be elucidated. The interest in gastroenteropancreatic neuroendocrine tumors arising from these cells has markedly increased in recent years as a result of the creation of precise biochemical techniques to confirm the diagnosis and the refinement of localization techniques that allow for the identification of previously occult tumors. Therefore, the recognition and treatment of gastroenteropancreatic neuroendocrine tumors has improved so that an ever-increasing number of patients are diagnosed early and surgically treated before metastases occur. There are still more identified gastroenteropancreatic neuroendocrine cells and humoral products than there are recognized neoplasms and syndromes; continued investigation is essential. Undoubtedly, newer treatment modalities will continue to be created for chemotherapy and receptor modulation, but early surgical excision remains the cornerstone of successful treatment today.