Inadequate cerebral blood flow (CBF) after head injury is an important cause of secondary ischaemic damage. Rapid identification of episodes of hypo- or hyperperfusion would allow timely intervention and would possibly improve outcome. Despite a large number of methods to estimate CBF, this concept is only marginally implemented in clinical practice. The methods to detect such episodes are limited for technical reasons, but also because the thresholds of ischaemia and hyperaemia are variable after head injury. Furthermore, we are not always able to manipulate CBF in a controlled manner. Accordingly, it is not surprising that attempts to compare a CBF-targeted strategy with another management option have failed to demonstrate a clear benefit. Methods need to be developed that allow either identification of thresholds for critically low or high CBF in individual patients, allow monitoring oxygen extraction fraction, representing circulatory reserve, or alternatively provide a measure of the volume of ischaemic or hyperaemic brain.