It is well recognized that during atrial fibrillation (AF), clots may form in the left atrium. This, in turn, may lead to embolization of the clot, with resulting ischemic stroke or systemic embolism. Also, the presence of AF confers a fivefold increased risk for stroke. AF is the most common and important cause of stroke resulting from any cause. This article considers the risks for and anticoagulation prophylaxis against embolic stroke in patients with AF.