Cerebral aneurysms are pathological dilatations of the arterial wall frequently located near arterial bifurcations in the circle of Willis. The most serious consequence is their rupture and hemorrhage into the subarachnoid space, with an associated high mortality and morbidity rate. Greater availability and improvement of neuroradiological techniques have resulted in more frequent detection of unruptured aneurysms. Because prognosis of subarachnoid hemorrhage is still poor, preventive surgery is increasingly considered as a therapeutic option. But treatment risk is often comparable to the yearly risk of rupture. Therefore, the best patient care would be to treat only those patients who are likely to rupture, which requires a better understanding of the process of aneurysm formation, progression, and rupture. Since to date there are no reliable non- invasive technique for in vivo quantification of aneurysmal flow patterns, we use our state of the art computational tools for patient-specific hemodynamic modeling. This book includes validation and sensitivity studies as well as investigations of possible associations between hemodynamics and rupture.