Positron Emission Tomography (PET) is an imaging technique that produces cross sectional images based on tissue biochemical and physiological processes. PET complements other anatomic imaging techniques such as x-ray CT and magnetic resonance imaging (MRI). Fundamental processes such as glucose metabolism, oxygen metabolism, and blood flow can be imaged and quantified with PET, in addition to many other processes of both clinical and investigative interest. PET is now emerging as a clinical tool in oncology and is useful in noninvasively grading tumors, in determining tumor activity and recurrence, and in monitoring the effects of a variety of therapeutic interventions with tumors. While most of the applications of PET in oncology to date have been in brain tumors, the technique is now being applied in tumor evaluations outside of the central nervous system.