Care of patients with traumatic brain injury (TBI) has evolved with improvements in pre-hospital medicine, neuroimaging and access to multidisciplinary expertise through the development of specialist neurotrauma services, together with advances in the understanding of the underlying pathophysiology. After traumatic injury, a reduction in cerebral perfusion pressure (CPP) due to either systemic arterial hypotension or increased intracranial pressure (ICP), and/or pathological changes in tissue architecture and function, may lead to a reduction in global or regional oxygen delivery. Systemic haemodynamic disturbances may be observed after TBI. Increased sympathetic activation occurs as a reflex response to raised ICP, after brainstem compression or medullary ischaemia. The Glasgow Coma Score (GCS) provides a robust starting point in assessing the severity of TBI and any subsequent improvement or deterioration. Significant hypovolaemia may be masked in the young or by systemic hypertension secondary to intense sympathetic stimulation after TBI.