Positron emission tomography (PET) is a non-invasive, quantitative imaging technique that can visualise biochemical, physiological and biological processes in vivo. It is the most specific and sensitive molecular imaging technique in man. In recent years, PET has emerged as a valuable tool for tumour diagnosis, disease staging and treatment monitoring. In particular, PET has great potential as an important component of the radiotherapy treatment process. Although CT is the current gold standard for disease staging, PET has the potential to refine and improve this, and to provide powerful biological information.