To benefit from modern radiation therapy characterized by highly conformal dose distribution and steep dose gradients, high quality imaging is required for precise tumor delineation. The standard imaging for radiation therapy planning (RTP) remains the acquisition of a dedicated planning computed tomography (CT). Recent advances in combined functional and structural imaging, e.g. hybrid positron emission tomography and CT (PET/CT) have led to progress in management of cancer patients. As PET/CT has become widely available it is increasingly being incorporated into routine RTP. PET/CT in RTP is a multidisciplinary task involving the expertise in radiation oncology, radiology, nuclear medicine and medical physics. Imaging protocols and tumor delineation methods for PET/CT in RT planning should be well-defined and rigorously applied to guarantee reproducibility. The most often used radiotracer for PET/CT in RTP is the glucose analog [18F]FDG, depicting cell metabolism with high sensitivity but low specificity as it is not a specific tumor tracer. PET-based RTP can improve local tumor control by reducing target volume and using dose escalation, with fewer side effects by restricting RT to [18F]FDG avid volumes. Applications of functional imaging are continuously evolving with development of novel PET tracers aimed at specific biological processes. As personalized medicine has become a true force in modern medicine, the individualization of treatment strategies in RTP will require functional imaging in the tailoring of RT for each patient. Novel approaches in radiation therapy delivery, such as dose painting using PET/CT, have been suggested as methods to reduce risk of local recurrence. Equally important, is minimizing radiation toxicity to normal tissue by using PET/CT with breath hold techniques. Furthermore, PET/CT is increasingly used to predict and evaluate response to RT to optimize treatment strategy.