Through diagnostic imaging and peptide receptor radionuclide therapy, nuclear medicine has earned a major role in gastroenteropancreatic neuroendocrine tumors (GEP NETs). GEP NETs are diagnosed fortuitously or on the basis of symptoms or hormonal syndrome. The functional tumor characteristics shown by radionuclide imaging allow for more accurate staging and treatment selection. Tumor grade helps determine which tracer should be selected. In the past, 111In-pentetreotide has been successful in well-differentiated (G1 and G2) tumors. However, PET/CT imaging with novel somatostatin analogs (e.g., 68Ga-DOTATOC, 68Ga-DOTATATE, 68Ga-DOTANOC, and 64Cu-DOTATATE) now offers improved sensitivity. 18F-fluorodihydroxyphenylalanine (18F-FDOPA) is another interesting radiopharmaceutical. 18F-FDOPA sensitivity is influenced by a tumor's capacity to take up, decarboxylate, and store amine precursors. 18F-FDOPA sensitivities are highest in ileal NETs and may also be helpful in insulinomas. A high uptake of 18F-FDG with a low uptake of somatostatin analog usually indicates poorly differentiated tumors (G3). Starting from these principles, this article discusses theranostic approaches to GEP NETs, taking into account both primary and metastatic lesions.