Gastroenteropancreatic neuroendocrine tumours (GEP-NETs) represent in clinical practice a diagnostic dilemma because they are often very small, located deeply within the retroperitoneum or in an extramucosal site in the gastrointestinal (GI) tract and, lastly, because they may be multi-sited. Modern digestive endoscopy offers a myriad of techniques, useful for localization, diagnosis and treatment (therapeutic endoscopy). The available tools include upper digestive endoscopy (esophagogastroduodenoscopy, endoscopic retrograde cholangiopancreatography), lower digestive endoscopy (ileo-colonoscopy), enteroscopy (push-type, intra-operative, capsule, double or single balloon), for examining the small intestine, diagnostic and interventional echo-endoscopy (EUS), with radial, linear and miniprobe equipment. This narrative review offers scientific support to affirm that endoscopy and EUS give imaging and diagnostic possibilities that are unbeatable in the localization of GEP-NETs both of the GI tract and the pancreas. Endoscopy is useful for localization, bioptic diagnosis and curative resection of small neuroendocrine lesions of the stomach, duodenum, colon-rectum and more recently of the jejuno-ileum. EUS associated with dedicated instruments, particularly high frequency miniprobes, is a valuable procedure in locoregional staging of lesions of the GI wall and can supply information which has a clinical impact on therapeutic options and prognostic value. EUS is still today the sole technique in a certain number of cases which provides a definitive diagnosis of pancreatic insulinoma and to detect and follow subcentimetric lesions of the pancreas in patients with MEN-1 syndrome. It should be used in all those cases where results from radiographic imaging or nuclear medicine techniques show negative or dubious.