The particular advantages of positron emission tomography (PET) technique are that it has higher sensitivity, higher resolution, and a higher quality of image than that found in conventional nuclear medicine. The possibility of quantification and the wide range of useful tracers have raised expectations of this new method. To date, most of the human PET cancer studies have been performed with [18F]fluorodeoxyglucose (FDG) or [11C]methionine. These are good imaging agents for tumours. However, more specific radiopharmaceuticals are required if other features of tumour metabolism are to be observed. [11C]Thymidine may prove to be a good tracer for quantitative measurements of tumour proliferation and [18F]misonidazole has been suggested for imaging of hypoxia.