Measurement of cerebral blood flow (CBF) in the animal laboratory and on the clinical service differs in methodology and to a certain extent in rationale. This review will include a brief discussion of experiments from our own laboratory that compare the regulation of CBF in fetal, newborn and adult animals, followed by a comment on the advantages and shortcomings of CBF measurements on the clinical service. I will attempt to put CBF measurements in some perspective. The principal disadvantage of measuring CBF alone, in either the laboratory or clinical situation, is that CBF is just one of a number of variables that determine the brain’s well-being. A CBF value has to be considered in the context of cerebral energy requirements, on the one hand, and blood metabolic substrate concentrations on the other. The real criterion of the adequacy of CBF in an hypoxic subject, for example, is the functional state of the brain. New techniques of neurological monitoring that promise to give the clinician direct access to the brain’s level of function and its metabolic energy status will be mentioned at the end of the review.