Following subarachnoid hemorrhage (SAH) from an intracranial aneurysm, ischemic deficits related to cerebral vasospasm still account for significant morbidity and mortality. Operative decisions and timing must be based on the presence of vasospasm and other complications of the hemorrhagic period. Transcranial Doppler sonography provides a noninvasive method for evaluating the status of the intracranial arteries following SAH. The method can, with good reproducibility, identify the patient likely to suffer symptomatic vasospasm, outline the progress of the disease, and serve as a guide to therapy. There are inherent errors produced by the anatomy of the intracranial tree and by peculiarities of the disease. Proximal vasospasm, distal vasospasm, defective autoregulation, and distal infarction with hyperperfusion add confusion to the velocity equation. In experienced hands, however, the method correlates well with the angiographic image of the vessels studied.