Over the past few years, significant progress has been made in our understanding of the biology and functional significance of somatostatin receptors (sst) on human tumors. Somatostatin analogs, such as octreotide, bind predominantly to sst2 and successfully control hormone hypersecretion in patients with acromegaly, islet cell tumors and carcinoids, and (temporary) control of tumor growth is often also seen. Furthermore, sst2 on tumors can be imaged in vivo after the injection of radionuclide-coupled octreotide. Targeted chemo- and radiotherapy, in which somatostatin analogs coupled to a chemotherapeutic agent or a radionuclide are selectively internalized by sst-positive tumors, are now being studied for their effect on tumor growth. Knowledge about the differential anti-tumor effects of the sst subtypes on tumor cells might have clinical significance after the development of new subtype-specific somatostatin analogs.