Atherosclerosis involving the cervical vessels, intracranial vessels, or the aorta is the most common cause of ischemic stroke. Occlusive lesions of small penetrating brain arteries cause small "lacunar" strokes, which account for about 20% of ischemic strokes. Emboli from a variety of cardiac sources, particularly atrial fibrillation, account for about 25%. Efforts to prevent and treat ischemic stroke are complicated by the variety of etiologies underlying it and the selection of antithrombotic or thrombolytic therapy appropriate to the particular etiology.