Regulatory peptides are small, readily diffusable and potent natural substances with a wide spectrum of receptor-mediated actions in humans. High affinity receptors for regulatory peptides such as somatostatin, substance P, vasoactive intestinal peptides, and cholecystokinin can be overexpressed in several human diseases, in particular in neoplasms, and represent therefore new molecular targets for cancer diagnosis and therapy. The availability of suitable regulatory peptide radioligands, which can be labeled with radioactive iodine or indium, makes peptide receptor scintigraphy a particularly useful new in vivo diagnostic tool, as seen with the example of somatostatin receptor scintigraphy (Octreoscan). The present article reviews the current in vitro data on regulatory peptide receptor expression in normal and diseased tissues, which represent the basis for the in vivo application of these molecules in nuclear medicine.