The majority of subarachnoid hemorrhages (SAH) are caused by ruptured intracranial cystic aneurysms. Imaging and autopsy studies have found that intracranial saccular aneurysms account for 3.2% of the population without coexisting disease, with a mean age of 50 years and no gender differences. Multiple aneurysms account for approximately 20-30% of the intracranial aneurysm population. The incidence of aneurysmal subarachnoid hemorrhage is 6 to 16 per 100,000. In North America, this translates into an annual prevalence of approximately 30,000. Most aneurysms, especially small ones, do not actually rupture. Deaths from ruptured intracranial aneurysms account for 0.4 to 0.6% of all deaths from disease. Almost 10% of patients die before coming to the hospital, and only 1/3 of patients recover well after treatment. Most aneurysms (approximately 85%) are located anterior to the cerebral base artery ring. Common sites include the junction of the anterior cerebral artery and the anterior communicating artery (junction), the junction of the posterior communicating artery and the internal carotid artery, and the bifurcation of the middle cerebral artery (bifurcation). Common sites in the posterior circulation include the tip of the basilar artery (top), the connection between the basilar artery and the superior or anterior inferior cerebellar artery, and the connection between the vertebral artery and the posterior inferior cerebellar artery. Fifty-four to 61% of patients with intracranial aneurysms are women. The ratio of women to men is close to or greater than 2:1 in ≥50 years of age. Most intracranial aneurysms are asymptomatic unless they rupture. Some unruptured aneurysms are symptomatic: these include headache (which can be severe, similar to that caused by bleeding from subarachnoid hemorrhage), loss of vision, cranial nerve disease paralysis (especially of the motor nerve), pyramidal fasciculus dysfunction, and facial pain. These are all caused by the occupying effect of the aneurysm. Additional emboli originating from within the aneurysm may also cause ischemic symptoms. Treatment of small incidentally detected internal carotid artery aneurysms is not recommended. Large symptomatic intracavernous aneurysms should be selected based on the patient’s age, the severity and progression of symptoms, and the treatment approach. The high risk of treatment and the short life expectancy in older patients must be considered in all patients. Asymptomatic aneurysms in older patients are appropriate for continued observation. Symptomatic subdural aneurysms of all sizes should be considered for treatment as soon as possible. Coexisting or residual aneurysms of any size found after subarachnoid hemorrhage need to be considered for treatment. Aneurysms at the tip of the basilar artery have a relatively high likelihood of rupture. Treatment decisions require consideration of the patient’s age, neurologic condition, and relative risk of repair. Given the relatively low risk of bleeding in unruptured aneurysms less than 7 mm found incidentally, observation and follow-up is a generally accepted treatment decision. However, patients younger than 50 years of age in this population require extra attention. Asymptomatic aneurysms in the 7-10 mm range require close monitoring, taking into account age, coexisting disease and neurologic status and relative treatment risk before treatment. For unruptured aneurysms that have not been treated with clamping or embolization, the following follow-up monitoring is recommended: We recommend that patients with unruptured aneurysms be followed up with CTA or MRA every 2 to 3 years, or every 2 to 5 years if the aneurysm is stable; however, it is reasonable to follow up with CTA or MRA after 6 months for newly diagnosed small aneurysms. If there is no significant change in the aneurysm at the six-month follow-up, the follow-up interval can be extended appropriately. Patients should avoid smoking and excessive alcohol intake, stimulants and prohibited medications, and excessive straining and Valsalva maneuvers.