Advances in medical imaging have greatly enhanced the speciality of radiation oncology by allowing more healthy tissue to be speared for better tumour coverage. Positron emission tomography (PET) with the glucose analogue [18F]-fluoro-2-deoxy-D-glucose (FDG) is a functional imaging method that has become widely used in oncology over the last decade. It has been rapidly incorporated in the staging and treatment planing of many patients with cancer in several anatomic sites such as non-small cell lung carcinomas. However, the initial data were controversial by the use of non dedicated PET units, the lack of patient immobilisation for radiation therapy, or the lack of image registration for fusion PET images with computed tomography (CT). The increased number of combined PET/CT units installed and the development of new isotopes that allow advances in biological and molecular tumour and healthy tissue imaging should lead to enhanced target definition for highly conformal radiation therapy. Such developments might also allow tumour viability or healthy tissue function to be imaged, which could be used during treatment as early indicators of tumour response or healthy tissue injury, possibly leading to a change in treatment strategy based on functional and biological imaging. The contribution of PET imaging advances using FDG or new tracers for treatment planing in the new era of image guided radiation therapy will be discussed in this review.