The transcranial Doppler ultrasonography (TCD) was introduced into the practice of medicine in 1986 and has been used extensively in a variety of in- and out-patient settings. TCD provides a quantitative assessment of cerebral hemodynamics and measures linear cerebral blood flow velocity (CBFV) in cm/sec. Due to its noninvasiveness and easy applications, TCD examinations have gained an important role in the very early phase, as well during the repetitive assessment of cerebrovascular diseases. This has led to a broad application of TCD in out-patients, in-patients, emergency and intraoperative settings. TCD allows interrogation of the cerebral circulation in a safe, noninvasive, repeatable and cost-effective manner. TCD within 24 hours of symptoms onset improves the early accuracy of stroke subtype diagnosis in patients with large artery atherosclerosis. Early detection may affect therapeutic strategies in patients with acute cerebral ischemia or extra- or intracranial lesions (symptomatic or asymptomatic). The unique ability of TCD to monitor CBFV continuously and detect the time course of changes is invaluable in the Intensive Care Unit, during carotid surgery, certain endovascular procedures, etc. TCD helps guide decision making to prevent or minimize the effects of stroke, brain injury and it is a significant addition to the armamentarium of medicine.