Fluoro-deoxyglucose (FDG) is a metabolic marker, which follows the same route into cells as that of glucose, and it can be radiolabelled with fluorine-18, 18F-FDG making it suitable for imaging with positron emission tomography (PET). The fact that rapidly proliferating cells such as tumour cells accumulate 18F-FDG more avidly than those with a normal turnover rate has given rise to its potential in oncology. The rationale and previous published uses of 18F-FDG in oncology are reviewed, together with the various analysis techniques and associated methodological difficulties.