PET imaging with the most widely available PET tracer, 2-deoxy-2-[(18)F]fluoro-d-glucose (FDG), is a powerful tool in the differential diagnosis of numerous neurologic and psychiatric disorders, particularly in early disease stages. It also plays an important role in the longitudinal evaluation of treatment effects and the depiction of disease courses. A selection of established and eligible application areas of FDG PET in neurology and psychiatry, such as different types of dementia, epilepsy, schizophrenia, and bipolar disorder, are reviewed in this article. A general methodology for clinical FDG PET examinations and typical diagnostic criteria and pitfalls are addressed.