Positron emission tomography (PET) is an imaging technique that provides quantitative, regional measurements of biochemical and physiological processes in living human subjects. In this editorial, we review PET's capabilities; we distinguish between its established, apparent, and unestablished but potential value in science and medicine; and we describe challenges that must be addressed to maximize its value in these endeavors. CAPABILITIES Positron emission tomography has the potential to provide information about a wide variety of local organ functions. In the brain, it has been used in efforts to make regional measurements of blood flow, glucose utilization, blood volume, oxygen utilization, the oxygen extraction ratio, the permeability-surface area product for water, acid-base chemistry, protein synthesis, dopamine synthesis, and characteristics of dopamine D 2 , benzodiazepine, and opiate receptors. In the myocardium, it has been used in efforts to measure the distribution of blood flow, glucose utilization, and oxygen utilization. In other organs, it