Positron emission tomography (PET) allows for the absolute measurement of regional tissue physiological, biochemical and pharmacological processes. This ability is a consequence of the nature of positron emission and the type of tracers which can be labelled with positron emitting radionuclides. The spatial resolution of state of the art scanners is in the order of 4 mm full width at half maximum (FWHM). However, due to statistical limitations, in practice normally a resolution of 7 to 8 mm FWHM is used. For the same reason, although there is no limit to the temporal resolution, studies usually require many seconds to several minutes to obtain a good signal to noise ratio. If a slow kinetic process is followed, studies can be extended to several hours in order to characterise the entire physiological process. A major advantage of PET is its flexibility. Several parameters (e.g., blood flow, oxygen consumption, glucose consumption, receptor density, etc.) can be measured in the same setting. Different aspects of a certain pathology can, therefore, be studied in the same patient.