Positron emission tomography (PET) is increasingly used clinically to provide functional information on disease processes, especially in oncology using the glucose analogue 2-[18F]fluoro-2-deoxy-D-glucose (18F-FDG). In the clinical setting it has become standard practice to use simplified imaging protocols compared to the often complex methods developed for research using PET. This is partly due to scarcity of resources but also for reasons of patient comfort and compliance, and not least expense and patient throughput. Fortunately the resulting loss in information can be justified to some extent on the grounds that in clinical PET it is usually relative rather than absolute metabolic rates that are of interest. Nonetheless, there remain unresolved questions of how best to perform quantification in clinical PET.