Accurate delineation of the gross tumour volume (GTV) in radiation therapy process is mandatory, especially when sophisticated techniques for radiotherapy delivery are used. Functional imaging, particularly PET and its fusion with anatomical modalities, such as PET/CT, promises to improve detection and characterization of disease. Quantitative accuracy in PET imaging is affected by several factors, which influence PET data so as to accurately delineate volume for radiotherapy planning purpose. Several strategies in using PET for target volume definition in radiotherapy treatment planning are being explored in the last years. In the present paper we give an overview of different techniques proposed to generate PET-based tumour volumes and the potential of their use in the radiation oncology process.