Today, positron emission tomography (PET) using F-18 Fluoro-deoxyglucose (FDG) is, when available, the most important nuclear medicine procedure applied to oncology. Nevertheless, 2 main reasons for the clinical use of somatostatin analogues labeled with single photon emitting radionuclides are: a) the low accuracy of PET-FDG in neuroendocrine tumors (NET); b) the expression of somatostatin receptors (sstr) in most cells deriving from so-called neuroendocrine dispersed cells. The latter forms the premise for the use of radiolabeled somatostatin analogues, and (111)In pentetreotide (Octreo-scan) in particular, in the diagnosis of NET and other pathological conditions, including some benign diseases. Alongside diagnosis, staging and follow-up of NET, somatostatin analogues, whether radiolabeled or not, can have a role in evaluating prognosis and predicting therapeutic efficacy in cancer patients. Interesting indications have emerged with radioguided surgery and in diagnosing the activity of disease in patients with Graves' disease (exophthalmos), sarcoidosis, and rheumatoid arthritis. The pathophysiological premises to imaging, starting from an analysis of cells expressing sstr, binding affinity of octreotide for sstr, in vivo uptake of Octreoscan in lesions expressing or not sstr are discussed, as is the possible role of quantitative receptor scintigraphy in improving diagnostic accuracy based on tumor expression of sstr.