Transcranial Doppler (TCD) is a non-invasive, cost-effective point-of-care tool to help evaluate and understand cerebral hemodynamics, using a 2 MHz frequency phased array ultrasound probe. TCD is especially useful in the neurocritical care patient, in which varying degrees of consciousness may cloud the intensivist’s ability to follow the neurologic exam. TCD can be used to evaluate and trend information on intracranial blood flow in a variety of conditions. TCD is especially useful in neurocritical care when evaluating for cerebral vasospasm, cerebral vasculopathy, and ischemic stroke. It also has broader utility when investigating the effects of systemic critical illness on the central nervous system. Examples of this include ICP monitoring in acute liver failure, dialysis disequilibrium, and cerebral circulatory arrest. This chapter focuses on the application and interpretation of TCD in adults with critical illness.