This article has addressed the rationale for the clinical measurement of CBF in the neuro-surgical ICU. The techniques that are currently available for measurement of CBF have been reviewed, and those that are particularly appropriate for ICU use have been highlighted. Several examples of the role of CBF monitoring in the management of ICU patients have been described, and these clinical situations are considered further in other articles in this issue regarding the management of patients with head injury and subarachnoid hemorrhage. These CBF monitoring techniques should be integrated with TCD and cerebral metabolism monitoring as described elsewhere in this issue. With further clinical experience and research and development in the area of neurosurgical critical care, CBF monitoring is almost certainly destined to become a routine and widely employed technique.