Peptide and growth factor receptors are frequently expressed in high density in human tumors. A particularly high abundance of receptors for the peptide somatostatin (SS) is found in neuroendocrine tumors of the pancreas and gastrointestinal tract. 90% of the carcinoids and a majority of islet cell carcinomas, including their metastases, have usually a high density of SS receptors. The number of receptors correlates with the degree of differentiation of the tumor. Several different SS receptor subtypes can be expressed by these tumors, the SSTR2 subtype being the most frequently and abundantly expressed. The somatostatin receptors in tumors are identified with in vitro binding methods, molecular biology techniques or in vivo imaging techniques; the latter allow the precise localization of the tumors and their metastases in the patients. Since SS receptors in human hormone-producing gastroenteropancreatic tumors are functional, their identification can be used to predict the therapeutic efficacy of octreotide to inhibit excessive hormone release. Undifferentiated tumors, such as atypical carcinoids, exocrine pancreatic carcinomas and most of the colorectal carcinomas, usually do not express SS receptors in the tumor itself, but, interestingly, may trigger the expression of SS receptors in the peritumoral veins, where SS may play a role in hemodynamic tumor-host interactions.