**Abstract**

Transcranial doppler (TCD) ultrasonography is a diagnostic technology for ascertaining numerous physiologic and pathologic phenomena by monitoring the direction and velocity of blood flow in intracranial vasculature. It is a noninvasive, point-of-care diagnostic tool that provides continuous and reproducible bedside data without the use of radiation or contrast agents. In this chapter, we will review principles TCD ultrasonography, and review clinical utility of TCD in aneurysmal subarachnoid hemorrhage (aSAH) and stroke as relate TCD monitoring. We will also review the advances in the clinical application of TCD in cerebrovascular diseases including robotic assisted TCD devices in PFO detection as well as clinical trials using TCD for early detection of large-vessel occlusive ischemic stroke.

**Keywords:** transcranial doppler, aneurysmal subarachnoid hemorrhage, ischemic stroke, aneurysm, treatment
