As with nearly all physiologic monitoring modalities available to anesthesiologists and critical care physicians, there is little evidence that interventions triggered by information acquired from patient monitors actually improve clinical outcomes. Nevertheless, there are convincing data demonstrating that critically diminished cortical blood flow decreases brain function and viability. For example, Sharbrough et al42 demonstrated at the Mayo Clinic that decreases in cortical blood flow to levels less than 20 mL × 100 g−1 × minute abolish spontaneous, electroencephalogram activity. Cortical blood flow to levels 15 mL × 100 g−1 × minute or less abolish somatosensory evoked potentials,34 and this level of oligemia, if maintained, can produce permanent brain injury.36 Therefore, it is rational (though unproven) to base therapeutic interventions upon measurements of cerebral blood flow (CBF), as well as other parameters which affect CBF (blood pressure, CO2, intracranial pressure). This article is divided into three parts. The first describes basic theories upon which CBF measurement, methodologies are based; the second explains different CBF measurement techniques; third, the application of different CBF measurement techniques for both research and clinical applications are discussed. Understanding CBF measurement techniques, both their application and individual shortcomings, is important because patients are subjected daily to anesthesia, cardiopulmonary bypass, carotid endarterectomy with temporary occlusion of common carotid artery blood flow, or craniotomy with brain retraction, all of which may profoundly affect CBF. Anesthetics, such as barbiturates and volatile anesthetics, alter brain metabolism and blood flow, thus affecting brain well-being. And, finally, patients endure various physiologic perturbations (e.g., hypoxemia, hypercarbia, hypocapnia, hypotension, hypertension, seizures), which significantly affect cerebral blood flow. Thus, an understanding of the advantages, limitations, and applications of various methodologies used to measure CBF would, in the authors' opinions, prove useful in the care of patients at risk for brain injury. Though the technical limits and reliability of cerebral blood flow methodologies play an important role in the interpretation of CBF data, only the essential elements of each methodology are discussed here. In addition, some CBF flow measurement techniques, because of their nature, are used in nonhuman research only (injection of radiolabeled microspheres, venous outflow methods), and will be examined more briefly in this article than those used in clinical research. Nevertheless, as C.S. Roy and C.S. Sherrington, two pioneers of cerebral blood flow measurement, state: We must on this account say more about the technology of our subject than would be necessary were the subject a simpler one.40