Positron emission tomography (PET) is a molecular imaging technique that allows for accurate measurements of specific tissue functions. Staging of cancer using 2-deoxy-2-[18F]fluoro-D-glucose (FDG) is the most widely used diagnostic application. As PET is able to measure functional changes quantitatively, interest is growing in its use for monitoring response to therapy. This review describes both biologic characteristics of FDG and methodologic issues regarding its use in the head and neck region. In addition, the potential use of FDG for predicting and monitoring response to radiotherapy is discussed. Finally, the potential of some other tracers for monitoring response is reviewed.