Arterial spine labeling (ASL) techniques have matured to the point that they can provide robust quantitative multislice measurements of cerebral blood flow (CBF) under most circumstances. These techniques provide better spatial and temporal resolution than positron-emission tomography (PET) and are entirely noninvasive, requiring no injections or radiation. The most obvious clinical application is in the evaluation of acute stroke, in which the primary pathology is a lack of CBF, precisely the quantity that is measured directly by ASL. The one major technical challenge that currently prevents more general application in the brain is the sensitivity to abnormally long transit delays.