Intracranial aneurysms are a relatively common disorder with an autopsy prevalence of 2% to 5% in the general population. Nearly half of these aneurysms become symptomatic during the patient's lifetime, usually presenting as subarachnoid hemorrhage (SAH). In North America, approximately 28 000 cases of aneurysmal SAH occur each year, mostly in adults. As opposed to the fusiform aneurysms that are encountered in the extracranial peripheral vasculature, intracranial aneurysms are typically saccular with a well-defined neck and sac distinct from the lumen of the parent vessel, frequently at proximal intracranial arterial branching points. Although the pathophysiology of intracranial aneurysms is controversial, they are thought to arise from defects in the muscularis media which may be congenital or acquired. Once these aneurysms have developed, conditions like hypertension and tobacco smoking may increase the risk of rupture, leading to SAH. Unruptured aneurysms are believed to bleed at varying rates according to multiple factors, including their diameter at the time of diagnosis. Although evidence suggests that intracranial aneurysms are less likely to bleed if they are less than 7 to 10 mm, both angiographic and direct intraoperative observational studies have demonstrated that even smaller aneurysms may rupture. About 40% to 50% of patients die within the first month as a result of the initial hemorrhage and its complications. Of those who survive, approximately 20% succumb to rebleeding in the ensuing 2 weeks (50% in 6 months) if the aneurysms are not treated, with the highest rate of recurrent hemorrhage (4%) during the first 24 hours after initial rupture.