Publisher Summary Neuroendocrine tumors (NETs) are rare tumors originating from enterochromaffin cells of the neuroendocrine system of the bronchial and gastrointestinal tracts. Nonfunctional NETs usually present with symptoms related to mass effects, including abdominal pain, jaundice and small bowel obstruction. Due to the presence of a variety of cells producing different hormones, the gastrointestinal tract may be considered the largest endocrine organ in the human body. Depending upon the specific production of peptides and amines produced by the different neuroendocrine cell types, along with their tissue-specific localization patterns, they all influence specific physiological functions. Somatostatin (SST) plays an important role in the regulation of the secretions of various organ systems, including the pituitary gland, the pancreas and the gastrointestinal tract. Five types of duodenal NETs can be distinguished as gastrinomas, somatostatinomas, clinically non-functioning NETs, duodenal gangliocytic paragangliomas, and poorly differentiated neuroendocrine carcinomas. Pancreatic NETs can be divided into functional and non-functional tumors. Insulinoma is the most prevalent functioning pancreatic NET, with an estimated incidence of 1–3 per million per year.