Positron emission tomography (PET) is a diagnostic imaging method used to examine metabolic function and disorders thereof. Dedicated ring systems of scintillation detectors measure the 511 keV radiation produced in the course of the positron emission from radiolabelled, metabolically-active molecules. In order to obtain PET images appropriate for research and clinical use, the different PET detector components, reconstruction methods and data processing procedures have to be optimised. This chapter introduces the basics of PET physics and technology.