The emergence of peptide receptor radionuclide imaging (PRRI) is highly regarded as a landmark in the diagnosis of tumours and other diseases through interaction with cell membrane receptors. The small size and low molecular weight of peptides, compared to proteins and antibodies, facilitate rapid access to target tissue and similarly, fast clearance from nontarget tissue. The resultant high tumour to background ratio provides a unique opportunity to map the distribution and assess the severity of a variety of tumours by planar and single photon emission computed tomography (SPECT) of In and Tc labelled peptides. The emergence of radiolabelled peptides for use with PET/ CT such as Ga, F and Cu, and the use of new receptor binding ligands such as the various DOTA compounds, have revolutionized PRRI and improved its diagnostic power beyond expectation. The inevitable move to utilize this novel technique to deliver peptide receptor radionuclide therapy (PRRT) followed swiftly by replacing diagnostic with therapeutic radionuclides such as Y and Lu. The main use of PRRI and PRRT remains at present largely restricted to neuroendocrine tumours (NET) expressing somatostatin receptors, but efforts to export these novel techniques to other tumours expressing the same or different receptors are in progress. The rapid proliferation of PRRI and PRRT over a relatively short period of time is remarkable, as is the expanding research aiming at using new peptides that target different receptors in different types of tumours. All these factors combine to make it mandatory to dedicate a supplement of the EJNMMI to this subject. This supplementary issue has been complied by a group of experts in the field, some of whom are pioneers and leaders in the European and international platform. In addition to providing a historical background, they aim at highlighting the state-of-the-art as it stands now by comparing their experience with that of their contemporaries in an effort to reach a consensus. Other contributors offer a glimpse of the available research and what the future holds in this exciting arena. A good starting point in such a comprehensive review is to understand the rationale for the use of radiolabelled peptides, and Koopmans and Glaudemans [1] explore the development of peptides and the criteria required for imaging purposes with an overview of existing radiolabelled peptides in oncological and other diseases. Fani and Maecke of the Basel group [2] look at the radiopharmaceutical development of radiolabelled peptides with its multiple steps of identification of a target, synthesis of a peptide, radiolabelling, in vitro characterization and evaluation followed by use in clinical studies in humans. A systematic review is provided for the development of peptide probes and chelators, as well as a description of labelling with various SPECT and PET tracers including Tc, In, I, Y, Lu, Ga, Cu and F. PET/CT using Ga peptides is the second biggest breakthrough since the introduction of PRRI, offering a combination of higher binding affinity, different receptor targeting and superb high-resolution imaging. Decristoforo et.al. [3] A. AL-Nahhas Department of Nuclear Medicine, Imperial College NHS Trust, London, UK