Positron Emission Tomography is a widely recognized clinical imaging modality for tumor detection and for monitoring the response of tumors to therapy. It has also proven to be a valuable clinical tool for determining whether the muscle of the heart is receiving an adequate supply of blood and for evaluating the efficacy of coronary bypass surgery and coronary “stent” placement. Less well known is PET’s use as a research tool, in both humans and animals, to explore the physiology of various organs in the body. These explorations permit unique in-vivo measurements of many poorly understood aspects of human physiology and biochemistry - measurements that have never before been possible. This presentation will try to give a basic understanding of the physics and engineering aspects of how PET works, why it is such a sensitive imaging technique, and why it is so valuable both clinically and for research.