The primary aim of managing patients with acute brain injury in the intensive care unit is to minimise secondary injury by maintaining cerebral perfusion and oxygenation. The mechanisms of secondary injury are frequently triggered by secondary insults, which may be subtle and remain undetected by the usual systemic physiological monitoring. Continuous monitoring of the central nervous system in the intensive care unit can serve two functions. Firstly it will help early detection of these secondary cerebral insults so that appropriate interventions can be instituted. Secondly, it can help to monitor therapeutic interventions and provide online feedback. This review focuses on the monitoring of intracranial pressure, blood flow to the brain (Transcranial Doppler), cerebral oxygenation using the methods of jugular bulb oximetry, near infrared spectroscopy and implantable sensors, and the monitoring of function using electrophysiological techniques.