Aneurysmal subarachnoid hemorrhage (SAH) is a bleeding into the subarachnoid space caused by a rupture of an intracranial arterial aneurysm. Aneurysmal SAH represents 5% of all stroke types and affects about 9 per 100,000 individuals annually. SAH occurs at a young age; half of the patients are younger than 50. The prognosis is poor as about a third of the patients will die within the first weeks, and another third will remain permanently disabled. Delayed cerebral ischemia (DCI) occurs in 25-40% of all patients with an aneurysmal SAH and is one of the most feared and least understood complications after SAH. It usually develops between 4 and 10 days after the initial hemorrhage and frequently leads to persistent neurological deficits or death. For many years, DCI was thought to be the result of cerebral vasospasm, which is an arterial narrowing occurring in many patients after SAH. It was traditionally thought that vasospasms precede or coincide with DCI. However, recent evidence has casted doubt upon the significance of vasospasm as the direct cause of DCI after SAH. In addition, all clinical trials to date targeting vasospasms have failed in improving the outcome of patients. As a consequence, there is currently no effective treatment for DCI. Delayed cerebral ischemia (DCI) is the most feared and least understood complication after SAH. Recent evidence shows that cerebral blood flow (CBF) in the brain is directly regulated by astrocytes. These cells cover the vessels with their endfeet, and can locally regulate the microcirculation.