The brain is an exquisitely sensitive organ, requiring a constant supply of blood, oxygen, and glucose to function. Cerebral blood flow is autoregulated to provide a near constant blood supply despite fluctuations in whole body physiology. The blood–brain barrier acts to ensure that the brain microenvironment remains tightly regulated. The pressure within the cranium must also be tightly controlled to maintain optimal cerebral perfusion and ultimately prevent herniation of brain parenchyma. Several physiological parameters can be monitored including intracranial pressure, cerebral oxygenation and metabolic stress and clinical use is increasing including in traumatic brain injury and subarachnoid haemorrhage patients.