Single time-point studies, following animal sacrifice, do not reliably reflect the status of transplanted islets and considerable efforts are now being devoted to the development of noninvasive islet imaging techniques for visualizing transplanted islets in vivo1-6. Islets are difficult to quantify in vivo, because they constitute less than 2% of total pancreatic mass, and the problem is compounded with transplants, because the number of islets is considerably less than in the native pancreas. Positron emission tomography (PET) is a well-established quantitative and noninvasive imaging modality5-14. With the PET reporter gene (PRG)/PET reporter probe (PRP) system, based on a mutant form of herpes simplex virus 1 thymidine kinase (HSV1-sr39tk), the PET signal is directly proportional to the enzymatic activity of sr39TK9-14. In this protocol, we describe in detail a method for reporter gene labeling of islets and quantitative scanning using the reporter probe 9-(4-[18F]-Fluoro-3-hydroxymethylbutyl)-guanine ([18F]FHBG). Islets expressing HSV1-sr39tk are transplanted into mice, either under the kidney capsule or into the liver, followed by injection of [18F]FHBG. PET imaging of phosphorylated, trapped, ligand enables quantification of transplant mass.