Transcranial Doppler (TCD) is the best non-invasive real time diagnostic tool for the evaluation of intracranial cerebral hemodynamic. The following disease or clinical states are commonly performed with TCD: intracranial atherosclerosis, ischemic stroke (thrombolysis, collateral circulation, right to left shunt, detection of microembolic signals), vasomotor reactivity, vasospasm after subarachnoid hemorrhage, intracranial pressure monitoring, cerebral circulatory arrest (brain death), and intraoperative monitoring. TCD can be used in variable clinical situations to provide real time physiologic information that is not obtainable with other neurovascular images.