Transcranial Doppler ultrasound allows measurements of blood flow velocity to be made from the basal intracerebral vessels. The major advantages of transcranial Doppler ultrasound are that it is non-invasive, relatively cheap, can be performed with portable machines, allows monitoring for prolonged periods, and has a high temporal resolution making it ideal for studying dynamic cerebrovascular responses. In addition it has recently been demonstrated that it can be used to detect circulating cerebral emboli; these cannot be detected by any other currently available imaging modality.