Radiation therapy (RT) has a prominent role in the treatment of locally advanced head and neck cancer. Image and biologically guided intensity modulated RT are becoming strongholds of state-of-the-art management with positron emission tomography (PET)/computed tomography (CT) as the preferred diagnostic tool in treatment planning. The procedures required in the workflow from diagnosis to treatment plan are complex and consensus on optimal image acquisition, reconstruction parameters and contouring methods remains to be established. In spite of this the potential of PET/CT-based treatment planning has been widely recognized and many large referral centres have adopted the technique in either a routine or an experimental setting. PET/CT with 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG) assists in selection of correct treatment goal and dose optimisation and increases the confidence of contouring process modifying treatment plan in most patients. For dose escalation and adaptive RT strategies PET may provide regional distribution of desired tumour characteristics such as hypoxic, metabolically active or rapidly proliferating sub-volumes. It is expected that within a few years PET/CT will be recommended for all patients presenting with stage III-IV disease considering the obvious benefits associated with more accurate volumetric presentation of primary and locoregional disease and the improved opportunities to conform and escalate RT dose in an attempt to improve therapeutic gain.