Positron emission tomography (PET) is a technique of transverse tomographic imaging in which detection of two photons emitted from the annihilation of a positron and an electron is used to reconstruct the distribution of a positron emitting isotope within an object. PET provides the capacity to quantitatively measure the local tissue distribution of a variety of radionuclides that are attached to compounds that distribute according to function. Although this technique has been used to measure multiple functions and receptors within the brain, one of the most widespread uses is the measurement of local cerebral glucose metabolism based on the deoxyglucose method. In this article, the application of PET to clinical disorders such as dementia, brain tumors, psychiatric disease, epilepsy, movement disorders, and stroke as well as to normal states such as aging are examined.