Positron computed tomography allows for the in vivo measurement of the regional tissue concentration of positron-emitting radionuclides such as 15O, 11C, 13N, and 18F. By using different tracers, a variety of metabolic processes can be quantitated. These include blood flow, oxygen utilization, glucose utilization, amino acid transport, blood volume, cation exchange, pH, and others. This review will contain three sections. First, the principles of positron computed tomography and the progress in instrumentation will be discussed. Second, tracer models will be analyzed. These models are vital in the extraction of physiological data from the measurements of activity. Finally, clinical studies will be evaluated in terms of new pathophysiological information obtained.