Historically, carcinoids have long been known to be a morphologically distinct class of rare intestinal tumors that behave less aggressively than the more common intestinal adenocarcinomas. It was not until much later that their endocrine nature was recognized. Some authors restrict the term carcinoid to intestinal endocrine tumors, whereas others include a large variety of neuroendocrine tumors. In the WHO classification of 1980, carcinoids were defined as tumors of the diffuse neuroendocrine system that are either benign or neoplasms with a more favored prognosis than carcinomas. They are characterized by a typical growth pattern, silver affinity, and positive immunohistochemical reaction with neuron-specific markers; and they can express different peptides and biogenic amines. Neuroendocrine tumors originating from endocrine glands (pituitary, thyroid, adrenals, pancreas) and highly malignant neuroendocrine carcinomas are excluded from the carcinoid group of neoplasms. For the natural history of carcinoid tumors several independent predictive parameters can be defined: size, site of origin, growth pattern, and hormone dependence. The number of neuropeptides and amines expressed by a carcinoid or the amount of biologically active neurohormones secreted are of no prognostic significance regarding malignant behavior. The carcinoid syndrome is a rare clinical entity that occurs with a prevalence of 1.6% for carcinoid tumors and almost only if liver metastases are present. The complex clinical symptoms are only partially explained by the secretion of serotonin and tachykinins.