A small percentage of aneurysms cannot be treated by traditional techniques. The majority of these are giant aneurysms, whose dimensions exceed 2.5 cm, which are particularly difficult to treat because of their anatomical characteristics. Clinically, they may present with compressive, ischemic or hemorrhagic symptoms. Treatment of intracavernous aneurysms and those of the pre- and intrapetrous segment of the internal carotid artery represent a challenge for the neurosurgeon and on occasion may not be suitable for clipping or endovascular techniques. This is true for aneurysms without a well-defined neck and fusiform aneurysms, for which extra-intracranial bypass using a graft of saphenous vein plays a fundamental role. The method used for bypass depends on whether there is sufficient collateral flow. The authors emphasize that it is not possible to identify patients who can be submitted to therapeutic occlusion of the internal carotid artery without the risk of late ischemic attacks or de novo aneurysm formation.