Multiple approaches are available to measure global and regional cerebral blood flow. The inert gas method of Kety-Schmidt provides the mathematical basis for essentially all of these methods with the exception of microspheres. The preferred method depends largely upon the type of information to be obtained. The preferred method depends largely upon the type of information to be obtained. The three essential problems in cerebral blood flow measurement are establishing reliable cerebral blood flow measurements in pathological processes, the relation of this flow to metabolism, and the clinical application of cerebral blood flow measurements in man for the investigation of a wide variety of significant disease processes including ischemia and intracranial hypertension.