Neuroendocrine tumors (NETs) are defined as epithelial cell tumors arising from cells with predominant ­neuroendocrine differentiation. The term NET has replaced the term carcinoid and is functionally equivalent to the term neuroendocrine neoplasm. NET is preferentially used in this chapter because of its familiarity. NETs can arise in many different organs and tissues. For the purposes of this chapter, we review the epidemiology, pathology, clinical features, biomarkers, imaging characteristics, and treatment of gastroenteropancreatic NETs (GEP-NETs).