The brain depends on a continuous flow of blood to provide it with oxygen and glucose needed to maintain normal function and structural integrity, thus cerebral blood flow is normally tightly regulated. A decrease in cerebral blood flow to ischemic levels may be tolerated for only minutes to hours, depending on the severity of the ischemia. If cerebral blood flow ceases completely, brain cell death occurs within minutes. A variety of conditions are encountered clinically, such as stroke or traumatic brain injury, where an actual or potential alteration in cerebral blood flow puts the brain at risk for ischemia and infarction. In this article, the physiology of cerebral blood flow will be presented as a basis for understanding cerebral blood flow regulation and the rationale for clinical interventions to optimize cerebral blood flow. Techniques currently available to assess cerebral blood flow and clinical situations in which cerebral blood flow is measured will be discussed. Clinical interventions will be presented briefly.