Monitoring cerebral oxygenation has been one of the main fields of interest in neurointensive care during the past few years. In fact it is strongly believed that restoring adequate cerebral oxygenation is the premise to maintaining the viability and restoring the function of the damaged CNS. Global monitoring provides an indirect estimation of adequacy of substrates supply to the brain. Local measurement of brain oxygen tension (ptiO2) is possible through a Clark electrode implanted into the cerebral parenchyma. The paper describes the physical basis of the monitoring, the pathophysiology of ptiO2 and its clinical use.