The ability of the cerebral vascular bed to undergo constriction or dilation in response to various stimuli is termed vascular reactivity or cerebral autoregulation. When the stimulus is the change in cerebral perfusion pressure (CPP) the vascular response is termed cerebrovascular pressure reactivity. When the stimulus is the change in PaCO2 the vascular response is termed carbon dioxide reactivity. The transcranial Doppler (TCD) provides information about changes in flow velocities secondary to changes in CPP or PaCO2. Although cerebral blood flow (CBF) is not directly measured by TCD, changes in velocity are correlated with flow if the vessel cross sectional area remains constant and has been used as a method of assessing autoregulation. Loss of autoregulation is common in patients with severe traumatic brain injury (TBI) and is associated with severity of injury and increase mortality. In these patients, CBF is affected not only by CPP but by the state of autoregulation as well. Knowledge of the state of autoregulation with TCD examination using several methods may permit individualization of therapy. So far, none of these methods has an established role in routine clinical practice, but may be used as a part of multimodal monitoring, giving information about the risk of ischemia and enhances the potential of early effective interventions to reverse pathologic states on an individual basis.