The goal of this application is to develop novel tools to follow the fate and function of transplanted stem cells, based on a powerful medical camera called the PET scanner. PET imaging, or positron emission tomography, allows doctors to visualize the biology of cells in living organisms, including patients. We have developed three ways to follow transplanted cells. In the first approach, distinctive changes of functions inside cells are probed using radioactive small molecules. In a second approach, antibodies are used to detect cells based on distinctive markers on the surface of the transplanted cells. In the third approach, the transplanted cells themselves are “marked” using genes that cause the cells to emit a signal detectable by the PET scanner. We have demonstrated examples of these approaches and how they can be implemented in the clinic.