The evaluation of cerebrovascular reactivity provides quantitative and qualitative information about cerebral hemodynamics in patients with external and internal cerebrovascular disease. Cerebrovascular reactivity is defined as the capability of the intracerebral arterioles to dilate; it may be diminished or exhausted in patients with insufficiently collateralized stenoses or with occlusions of the arteries supplying the brain. Its evaluation by different tests may provide important informations about therapeutic approach.