Transcranial Doppler sonography can be used to evaluate a spectrum of intracranial and extracranial vascular abnormalities. It is of proved value in the detection and follow-up of vasoconstriction caused by subarachnoid hemorrhage and can be used to demonstrate significant stenosis or occlusion of basal intracranial arteries and coexisting routes of collateral circulation. Transcranial Doppler sonography can play an important role in the determination of brain death and can be used to identify the nidus of an arteriovenous malformation, along with its major routes of supply and drainage. The technique may provide insights into cerebral hemodynamics following trauma, stroke, or migraine. Use of transcranial Doppler sonography enables a rapid, noninvasive diagnosis of the subclavian steal syndrome, and it is a valuable adjunct to duplex carotid sonography for determining the effect of atherosclerotic lesions of the internal carotid artery on cerebral hemodynamics. In the operating room or angiographic suite, transcranial Doppler sonography can be used to monitor patients undergoing surgical, interventional, or diagnostic procedures for the development of cerebrovascular complications.