ositron emission tomography (PET) is one of the one of the most sensitive methods used to diagnose tumors, and to study brain functions in general. PET as we know it today started in the early seventies. One can distinguish ~5 generations of PET scanners since the 1976. All PET cameras as of now use scintillation detectors and some focus will be given to this. Most commercial systems of today combine PET and CT (x-ray tomography). The CT images provide anatomy and the PET attenuation and scatter corrections. For the future, spatial resolution and sensitivity is expected to be improved. Sensitivity may be increased by increasing the solid angle and, in addition, by utilizing time-of-flight information. The spatial resolution can, at the present technology level, be improved by using smaller detector elements.