AbstractTranscranial Doppler sonongraphy is a non-invasive, non-ionising, inexpensive, portable and safe technique that uses a pulsed Doppler transducer for assessment of intracerebral blood flow. This article deals with the principles and technique of transcranial Doppler sonography. It gives a brief overview of its use in evaluation of intracranial steno-occlusive disease, subarachnoid haemorrhage, and extracranial diseases including carotid artery disease and subclavian steal syndrome. The role of transcranial Doppler in detection of microembolic signals and evaluation of right to left shunts is also dealt with. Finally, its use in acute stroke is briefly outlined.