**1. Introduction**

Transcranial doppler (TCD) is a non-invasive ultrasound technique that allows for the assessment of cerebral blood flow velocity in real-time without the use of iodine radiation commonly used in neurocritical care. It is an important bedside tool in the evaluation of cerebrovascular diseases including aneurysmal subarachnoid hemorrhage (aSAH) and ischemic stroke. In this chapter, we will review the principles of transcranial doppler, and the clinical application of TCD in aSAH for monitoring vasospasm and microemboli detection in large-vessel vasculopathy. We will also review the advances in the clinical application of TCD in cerebrovascular diseases robotic assisted TCD devices in PFO detection as well as clinical trials using TCD for early detection of large-vessel occlusive ischemic stroke.
