Positron emission tomography (PET) is a technique that permits the noninvasive in vivo measurement of a variety of cerebral physiologic and biochemical processes. Numerous complex methodologic and technical features must be considered when PET is applied to the study of the newborn's cerebral physiology and neurologic disease. Although the application of PET to newborn infants is still in its formative stages, important information regarding perturbations of cerebral blood flow in neurologic diseases has been obtained and striking developmental changes in cerebral metabolism are apparent.