Abstract: PET is a functional imaging modality, which can give some biological information of tumor. PET is more and more important in the definition of target volumes and radiotherapy treatment planning. Depending on its sensitivity and specificity, ¹⁸F-fluorideoxyglucose ¹⁸F-FDG PET has been shown to influence the selection of target vofumes and radiotherapy treatment planning for non-small cell lung cancers, for head and neck squamous cell carcinomas or for esophageal tumors. On the other hand, for tumors such as rectal carcinomas, convincing data on the value of ¹⁸F-FDG PET for target volume selection are still lacking. However, the application of ¹⁸F-FDG PET in many aspects of radiotherapy is still controversy. Further researches in its clinical application are still needed to investigate whether ¹⁸F-FDG PET for treatment planning should be routine because of the lack of prospective studies.