Positron emission tomography is an imaging technique based on the use of radiopharmaceuticals. The most extended one is the glucose analogue 18F fluorodeoxyglucose (FDG) that is deposited where there is an increase of glycolytic metabolism, whether this is caused by neoplastic, inflammatory or infectious diseases. It is used in oncology for the initial staging, to assess response to treatment, residual disease, recurrent diagnosis and restaging, but specifically among the different types of tumor. It also has a field in the study of large vessel vasculitis, in granulomatous diseases and in dementias.