Monitoring cerebral blood flow (CBF) continues to be a long-standing challenge in the neurocritical care unit. This chapter outlines the methods most commonly employed for the measurement and estimation of CBF in the operating theatre and in intensive care. CBF measurements obtained by Kety-Schmidt method are global and it is not possible to discriminate between grey and white matter or to detect changes in regional CBF. The introduction of radioisotope techniques for the measurement of CBF has allowed the progression from global CBF measurements to the two-dimensional maps of cortical blood flow. CBF can be measured by the exponential pattern of clearance of the gas from the brain. Sampling from the right jugular bulb has commonly been assumed to provide the best estimate of hemispheric blood flow. Jugular thermodilution technique, first used to measure coronary sinus flow, has been successfully adapted to measure CBF with reasonable accuracy.