The introduction of positron emission tomography (PET) as a powerful imaging modality has played a major role in the understanding of the pathophysiological bases for cerebrovascular disorders. PET is the only technique that allows measurement of regional cerebral blood flow, blood volume, oxygen extraction fraction, and oxygen and glucose metabolism with detail and accuracy. Using PET, these physiological parameters can be measured to determine the extent of the disease from the early stages of cerebrovascular disorders to acute cerebral infarction. Significant hemodynamic and metabolic abnormalities are noted in chronic ischemia, but no structural changes are noted on anatomic images. PET studies have shown that in many patients in the early phases (10 to 12 hours) of clinically diagnosed acute stroke, a substantial area of ischemia exists, which, if untreated, will become irreversibly damaged. Similar to the results achieved in patients with acute myocardial infarction, appropriate intervention in patients with cerebrovascular disorders may significantly reduce the extent of injury to the brain. PET also has been useful in predicting functional recovery and monitoring the effects of various therapeutic approaches. Although functional imaging of the brain with single photon emission computed tomography can successfully be used in the investigation of several disorders of the brain, its role in cerebrovascular disorders is quite limited. PET is a unique modality that studies ischemic diseases of the brain, and it potentially could play a significant role in the management of patients with cerebrovascular disease. This will be further realized when aggressive approaches are used routinely in the future.