A transient ischaemic attack is "an episode of focal neurological dysfunction of ischaemic origin, with a sudden onset, which resolves entirely in less than 24 hours". This 24-hour limit is actually only theoretical: a "real" transient ischaemic attack recovers in less than half an hour. More prolonged transient ischaemic attacks share the same mechanisms and the same prognosis as rapidly regressive constituted ischaemic accidents. The transient ischaemic attack constitutes a precious warning sign, indicating the need for systematic assessment looking for the cause of this accident and appropriate preventive measures depending on the results of this assessment: treatment of vascular risk factors, platelet antiaggregants, oral anticoagulants, carotid endarterectomy. These measures can reduce the risk of development of cerebral infarction, estimated to be 25% during the 5 years following the transient ischaemic attack in non-treated patients.