Each aneurysm arises from the branching site of a large artery. Most are located on or near the circle of Willis. More than 90% are located at one of the following five sites: (a) the internal carotid artery at the level of the posterior communicating artery; (b) the junction of the anterior cerebral and anterior communicating arteries; (c) the proximal bifurcation of the middle cerebral artery; (d) the junction of the posterior cerebral and basilar arteries, and (e) the bifurcation of the carotid artery into the anterior cerebral and middle cerebral arteries. Other aneurysm sites on the carotid artery are at the origins of the ophthalmic, superior hypophyseal, and anterior choroidal arteries. Other sites on the vertebral and basilar arteries include the sites of origin of the anteroinferior cerebellar, posteroinferior cerebellar, and the superior cerebellar arteries and the junction of the basilar and vertebral arteries. (Images courtesy of AL Rhoton, Jr.)