Type 1 and 2 Diabetes Mellitus are a growing health problem throughout the world. There is an increasing  need for methodologies, which are both reliable and non-invasive to measure the amount of insulin-producing tissue (Beta-cell mass, or BCM), as well as rapidly quantify changes in the BCM due to the onset of disease, beta-cell replacement therapy, or other treatments. Positron Emission Tomography (PET) is a non-invasive, quantitative functional imaging technique which can be used to study dynamical or static processes inside the body. In this thesis, we present a study protocol for in vivo imaging of the most common form of beta- cell replacement therapy; islet transplantation. Islets were labeled with the PET tracer, 2-deoxy-2[18F]fluoro-D-glucose ([18F]FDG), and administered intra-portally, while the recipient was monitored by PET/CT. The hepatic distribution of the islets was highly heterogeneous, and around 25% (human) or 50% (porcine) of the administered islets could not be found in the liver after completed transplantation, confirming previous reports of considerable cell injury during the procedure leading to low hepatic engraftment. Native BCM in the pancreas can potentially be quantified using a PET tracer with sufficiently high specificity, but the major obstacle is the relative low amounts of insulin producing tissue (only 1-2% of the pancreatic volume). Two tetrabenazine analogues, [18F]FE-(+)-DTBZ and [18F]FE-(+)-DTBZ-d4, are ligands to VMAT2, which is expressed in islet tissue. Both analogues were investigated and characterized as potential BCM imaging agents both in vitro and in vivo.  Both tracers exhibited high preferential binding to islet tissue compared to exocrine pancreatic tissue. However, the specificity was not high enough to overcome the obscuring exocrine signal in vivo (7-10% of the signal originating from specific islet tracer uptake). This thesis demonstrates that it is possible to quantitatively assess islet transplantation by PET imaging. In vivo determination of native pancreatic BCM is, in theory, possible with both [18F]FE-(+)-DTBZ and [18F]FE-(+)-DTBZ-d4, but tracer analogues with higher islet specificity is needed for quantification of smaller BCM changes with physiological impact.