The frequency of cerebral aneurysms is difficult to ascertain because of variation in the definitions of the size of aneurysm and modes of detection. Autopsy series cite prevalences of 0.2-7.9%. Prevalence ranges from 5-10%, with unruptured aneurysms accounting for 50% of all aneurysms. Pediatric aneurysms account for only 2% of all cerebral aneurysms. In the United States, the incidence of ruptured aneurysms is approximately 12 per 100,000 individuals or 30,000 annual cases of aneurysmal SAH. The frequency of cerebral aneurysms has not declined in recent years. [22] Saccular (berry) aneurysms constitute 90% of all cerebral aneurysms and are usually located at the major branch points of large arteries. Saccular aneurysms frequently rupture into the subarachnoid space, accounting for 70-80% of spontaneous subarachnoid hemorrhage. Annually, 15,000 American patients have SAH from aneurysms with a maximum diameter <7 mm and consequently experience irreparable morbidity and severe mortality. The majority of their aneurysms were unruptured, single, asymptomatic, and even smaller at some point before rupture [23]. Fusiform or arteriosclerotic aneurysms are elongated outpouchings of proximal arteries that account for 7% of all cerebral aneurysms and infectious or mycotic aneurysms usually situated peripherally comprise 0.5% of all cerebral aneurysms. Small aneurysms are less than 10 mm in diameter. Larger aneurysms are 10-25 mm and giant aneurysms are greater than 25 mm in diameter [24]. About 80-90% of the aneurysms are small and only 10-20% are large and giant [5].