Cerebral vasospasm is the delayed, usually reversible narrowing of large intracranial arteries in the days after aneurysmal subarachnoid haemorrhage (aSAH). Delayed cerebral ischaemia (DCI) is the broader syndrome of deficit or infarction arising in this period; it is multifactorial, combining large-artery spasm with early brain injury, microcirculatory dysfunction, microthrombosis, spreading depolarisation and inflammation. Detection relies on clinical examination, supplemented in poor-grade patients by transcranial Doppler, CT angiography and perfusion imaging, with catheter angiography as reference. Prevention is oral nimodipine; haemodynamic augmentation, intra-arterial vasodilators and balloon angioplasty are used when deficits persist, and European guidance offers only expert-consensus statements, not evidence-based recommendations, on induced hypertension and endovascular rescue.