The use of positron-emission tomography (PET) in the treatment planning process has become more and more popular over the years, although important questions such as how, when, and for which clinical benefit have never been answered. In this framework, the objective of this article is to review the evidence supporting the use of PET in radiotherapy treatment planning, with special emphasis on its application for head and neck tumors. The use of positron-labeled fluorodeoxyglucose for target volume selection should be discussed in terms of sensitivity and specificity in comparison with typical anatomic imaging modalities. It will not be of similar utility across all tumor sites. The use of PET for target volume delineation requires specific tuning of parameters such as image acquisition, processing, and segmentation, and this may vary from one tumor site to another. Molecular imaging with other tracers and "theragnostic" are in the pipeline, but how much the patient will gain from it and how these advances should be implemented in routine clinical practice are unresolved questions. Therefore, although integration of PET images into the radiotherapy process seems promising, for the moment it should remain in the research arena.