Imaging is playing an increasing role in the management of radiotherapy patients in terms of diagnosis and treatment planning, thus providing a stimulus for an expansion in its parallel development as a tool for imaging tumour biology in man. Positron emission tomography can be used to image not only glucose metabolism but also different biological processes involved in radiotherapy resistance such as hypoxia, blood flow and proliferation. It has potential for increasing our understanding of the biological mechanisms underlying the heterogeneity of tumour and normal tissue response to radiotherapy. It also has potential for the future definition of biological target volumes in tumours for dose escalation, and in normal tissues for dose reduction. © The British Institute of Radiology.