The cerebral vasomotor reactivity (VMR) indicates the ability of the cerebral arterioles to change their vascular tone under external stimuli. Greatest influence on the VMR exert age, endothelial functions and blood rheological properties. The most frequent influences are inhalatory induced changes in the partial pressure of carbon dioxide and infusion of acetazolamide. The alterations in the cerebral circulation are examined mainly with transcranial Doppler sonography or magnetic resonance imaging. In patients with carotid stenoses and cerebral infarctions and also in other diseases the estimation of the VMR is  important for evaluating the pathogenetic mechanisms and the clinical outcome and for selecting the therapeutic behavior in these patients.