Positron emission tomography (PET) has been used clinically to measure enzyme reactions, ligand-receptor interactions, cellular metabolism and cell proliferation. Until recently, however, PET has not been suitable for small animal models because of resolution limitations. Development of micro-PET instrumentation for small animal imaging and the availability of positron-emitting tracers has made this technology accessible for the non-invasive, quantitative and repetitive imaging of biological function in living animals. The development of new probes and positron-imaging based reporter genes has extended micro-PET applications to investigations of metabolism, enzyme activity, receptor-ligand interactions, protein-protein interactions, gene expression, adoptive cell therapy and somatic gene therapy. Because small animal PET is immediately extrapolatable to the clinic, laboratory advances should rapidly be translated to clinical practice.