Transcranial Doppler (TCD) evaluation is dependent on the accurate identification of intracranial vessels. Manual extracranial vertebral artery compression was used to modulate blood flow in vessels of the vertebrobasilar system in 118 patients examined by TCD. Two examples of its utility are presented in detail. Complete examination by TCD should include evaluation of the posterior cerebral circulation and its collaterals; vertebral artery compression is a new and important adjunct to the complete evaluation of intracranial vascular anatomy and cerebral collateralization.