Interest in cerebral revascularization surgery has recently been re-kindled in an effort to reduce the risk of ischemic stroke in selected patients with cerebral hemodynamic insufficiency. Patient selection is of utmost importance, and requires an understanding of the conditions where bypass surgery is used and the different available modalities for assessment of the cerebral hemodynamic status. SPECT can play an important role in selecting patients for surgery. It is able to provide an indication of cerebral hemodynamic insufficiency, is widely available, and has been extensively studied in this setting. Cerebral hemodynamic status is a critical factor in determining the need for, and type of, cerebral revascularization surgery. SPECT is one of several methods used to evaluate cerebral hemodynamics. This chapter discusses the conditions where bypass surgery is used and the role of SPECT in selecting patients for surgery.