To better characterize neuropeptide Y (NPY) receptor subtypes, several novel selective analogues of the NPY sequence have been developed in the last few years. These analogues show high selectivity towards single NPY receptors and facilitate insights into the localisation and physiological role of each receptor subtype in health and disease. Besides the important role of NPY in the regulation of food intake and other physiological functions, recently NPY receptors have been identified to play an important role in several types of cancer. To further enlighten all these numerous functions the radioactive labelling of peptides is still an indispensable and versatile tool. By using the appropriate radioisotope and labelling strategy several different radioisotopes can be incorporated into the sequence of subtype selective NPY analogues. Depending on the radioisotope the resulting radiolabelled NPY analogues can be used for diagnostic or therapeutic applications and enable us to better investigate single receptor subtypes in vitro and in vivo.