Cerebrovascular reserve (CVR) can be assessed by measuring the hemodynamic response to a physiological stress such as alteration of blood pressure, increase in tissue acidosis, lowered oxygen supply, increase in metabolic demand, or occlusion of an artery. Failure of the cerebrovascular system to maintain function or normative values of several interrelated hemodynamic variables--cerebral blood flow (CBF), oxygen extraction fraction (OEF), cerebral blood volume (CBV), and cerebral metabolic rate of oxygen (CMRO2),--in response to a stress implies a compromise of the normally robust compensatory mechanisms. The conclusions that are possible from this information depend on the type of stress induced and the technology used to measure the response. Technologies that permit a rapid test-retest format coupled with a physiological stress provide the most direct information about the hemodynamics of cerebrovascular territories. Patients whose cerebral vasculature becomes compromised by any of a broad range of disorders and who, thus, are at increased risk for stroke now can be readily identified based upon evidence of exhausted CVR. Strategies for treating hemodynamically driven disorders also can now be designed based upon such patient-specific CVR information. It is hoped that integration of CVR into the standard clinical assessment of patients with occlusive vascular disorders (OVD) will lead to treatments that focus not only on the previously understood embolic causes of stroke, but also on the often interrelated hemodynamic factors.