Transcranial Doppler sonography has become widely used in assessing cerebral vasomotor reactivity which provides information regarding cerebral autoregulation and collateral circulation. Cerebral vasomotor reactivity is defined as a shift between cerebral blood flow or cerebral blood velocity before and after administration of a potent vasodilatory stimulus test. Three such tests are currently used for this purpose: the apnea test, CO2 inhalation and the Diamox test (i.v. acetazolamide), all of which are based on the dilatatory response of cerebral blood flow to hypercapnia. Certain advantages of the Diamox test were described, but each of the three tests has its strong and weak points, and this will be the topic under discussion. There are several practical applications of the combined TCD and the vasodilatory tests in assessing of cerebral vasomotor reactivity: 1. To evaluate the intracranial hemodynamic status in patients with carotid occlusive disease with the intent of predicting the occurrence of ischemic brain events. 2. To compare intracranial hemodynamics before and after carotid endarterectomy. 3. To compare autoregulation and collateral circulation in the different parts of the circle of Willis. 4. To predict dementia after stroke. In summary, the potential clinical usefulness of combined TCD and provocative vasodilatory tests has been clearly shown. Further large scale studies are needed in order to augment the applications of cerebral vasomotor reactivity assessment in the clinical setting.