Positron emission tomography (PET) allows high-resolution, three-dimensional evaluation of regional brain metabolic activity and neurotransmitter function. This imaging technique has been applied increasingly in psychiatric research and may yield new information regarding the neural mechanisms of several mental disorders. This article is an overview of PET studies conducted on schizophrenia, affective disorders, and anxiety disorders. Although this research is in the preliminary stage with some inconsistent findings, general trends have emerged that clearly warrant further investigation. These trends are discussed in light of relevant methodological and theoretical issues. Potential directions for future PET research are reviewed.