Neuroendocrine tumors (NETs) are neoplasms that arise from cells of the endocrine and nervous systems. Many are benign, while some are cancers. They most commonly occur in the intestine, but are also found in the lung and the rest of the body. Although there are many kinds of NETs, they are treated as a group because the cells of these neoplasms share common features, such as looking similar, having special secretory granules, and often producing biogenic amines and polypeptide hormones.