Neuroendocrine tumors of the gastroenteropancreatic system present both a diagnostic and therapeutic challenge. Even in a patient presenting with a clinical hypersecretion syndrome such as the carcinoid syndrome, the Zollinger-Ellison syndrome or the hypoglycemia syndrome the primary tumor is often difficult to localize. Recently, tumor imaging has been improved by two new powerful techniques, the endosonography and the somatostatin-receptor scintigraphy. In addition, the new arterial secretin or calcium injection tests may guide surgery in patients with gastrinoma or insulinoma, respectively. The recently introduced positron emission tomography with nC-5-hydroxytryptophan can provide information about biochemical and metabolic changes in neuroendocrine tumors. Patients with the hereditary form of neuroendocrine tumor disease of the foregut suffer from the multiple endocrine neoplasia type 1 syndrome. The genetic defect of this autosomal dominant disease has been assigned to chromosomal region 11 q13. Current investigations aim to detect the multiple endocrine neoplasia 1 gene and thus to allow early diagnosis of hereditary neuroendocrine tumor disease of the foregut.