Neuroendocrine tumors (NETs) originate from neuroendocrine cells which are located throughout the body. This chapter focuses on low-to intermediate-grade NETs of the gastrointestinal tract, though the term neuroendocrine tumor can also denote other diseases such as small-cell carcinoma of pulmonary and extrapulmonary origins, thyroid medullar carcinoma, neuroblastoma, and Merkel cell tumors. Islet cell carcinomas, also known as pancreatic endocrine tumors, pancreatic NETs, or pancreatic carcinoid, arise from the islets of Langerhans. Low-to intermediate-grade NETs arising from other sites are generally called carcinoids and are most commonly localized in the gastrointestinal tract and bronchopulmonary tree. Both of these tumor groups share the capacity for hormone production and usually have indolent clinical courses. Presenting symptoms, when present, are caused by excess hormones, local tumor growth, and metastasis. Surgical resection is the curative approach for localized disease. In unresectable, metastatic disease, the introduction of somatostatin analogs such as octreotide have significantly improved quality of life, and the potential tumor stabilization properties of newer, targeted agents are the subject of ongoing phase III trials. Nonetheless, advanced NETs remain largely incurable and often require the concerted efforts of a multidisciplinary team for effective palliation. © 2011 Springer-Verlag Berlin Heidelberg.