Both ruptured and unruptured aneurysms are candidates for endovascular therapy. The natural history of unruptured intracranial aneurysms is still unclear and is influenced by many factors such as previous subarachnoid hemorrhage from another aneurysm, history of cigarette smoking, coexisting medical conditions, and aneurysm characteristics such as size, location, and morphology [25]. In a Study of the natural history of aneurysms to determine the risk of rupture, Juvela [26] followed 142 patients with 181 unruptured aneurysms for a mean of 13.9 years (range 0.8 - 30 years) to death or subarachnoid hemorrhage. The annual rupture rate was 1.4%. The median diameter of the aneurysm followed was 4 mm at diagnosis. Fourteen of the intracranial hemorrhages (ICH) were fatal; the authors concluded that unruptured aneurysms should be treated if technically feasible irrespective of size.