Positron emission tomography (PET) is emerging as a very useful clinical tool and is adding a great deal to our understanding of the pathophysiology of central nervous system (CNS) disorders. Although computed tomography (CT) and magnetic resonance imaging (MRI) have had a dramatic impact on patient management, there is often an important associated function abnormality which is best assessed by PET. In normal aging and in dementia, the CT and MRI brain changes of atrophy and white matter abnormalities are frequently nonspecific. PET has been more diagnostic, showing characteristic regional metabolic abnormalities. Evaluation of brain tumors such as astrocytomas with PET has demonstrated better correlation with histologic grade compared to CT. Unlike CT or MRI, PET can help to distinguish radiation necrosis from recurrent tumor, and can differentiate the extent of metabolically active tumor from surrounding edema. PET is useful in evaluating stroke patients, providing better prognostic information and demonstrating abnormalities sooner than CT. In epilepsy, PET appears to be superior to MRI in localizing seizure foci in patients with partial seizures. In head trauma patients, metabolic patterns are being described which will likely have an effect on patient management. The use of PET in schizophrenia has yielded very interesting results, with common patterns of metabolic abnormalities being demonstrated. CT and MRI in these patients have not been very useful. PET has also shown promise in movement disorders such as Huntington's disease. It is now clear that PET is already clinically useful and can provide valuable information unobtainable by CT and MRI. As new radioligands are developed, PET is certain to assume an even more important role in the future.