Positron emission tomography (PET), especially F-18 fluorodeoxyglucose (FDG)-PET, has been recently used to verify the target volume in radiation treatment planning (RTP) for malignancies. The utility of FDG-PET/CT in defining gross tumor volume (GTV) has been shown in many studies, and the target delineation by a fixed threshold of the maximum standardized uptake value (40-50%) is suggested to be useful in RTP for lung cancer, head and neck cancer, etc. But, the spatial resolution, sensitivity, and specificity of PET are not always enough to define the difference between the GTV and the clinical target volume(CTV). Furthermore, FDG-PET is frequently used in the clinical staging before the treatment, and is also applied to the response evaluation after the treatment. This review focuses on the developing applications of PET in radiation oncology.