Positron-emission tomography (PET) has been used extensively in the clinic for cancer diagnosis, for staging and for monitoring of therapeutic efficacy. PET has not, however, been used extensively in contemporary animal cancer models. Until recently, appropriate instrumentation was not available and the expertise and knowledge necessary to perform PET analyses in murine models has not been widespread. The fabrication of microPET instruments with appropriate resolution for murine experiments has lead to the establishment of non-invasive techniques for functional imaging. The development of "PET reporter genes" whose activity can be monitored in living animals, based on the reporter gene-dependent sequestration of positron-emitting "PET reporter probes," has lead to innovative analyses of gene expression in transgenic animals, to methods to monitor the location, magnitude and duration of expression for gene therapy vectors and to the ability to non-invasively track the targeting, viability and expansion of cellular therapeutics.