By the late 1970s, nonvalvular atrial fibrillation was identified as an independent risk factor for stroke (1–4), and millions of people with this common cardiac dysrhythmia were designated as having substantial risk for cardiogenic embolism (5). One of every six strokes occurs in a patient with atrial fibrillation, and about 10% of all ischemic strokes are probably due to embolism of left atrial thrombi. In the past decade, randomized clinical trials assessed the efficacy of antithrombotic therapies for stroke prevention in patients with nonvalvular atrial fibrillation. Eighteen trials involving more than 10 000 participants with atrial fibrillation have compared anticoagulants and antiplatelet agents, alone and in combination, with placebo and with each other (6). The remarkable pace of development has revolutionized management of this long-neglected dysrhythmia and makes detection of atrial fibrillation an important opportunity for stroke prevention. We present recent advances, discuss controversies, and explore new ideas about stroke and its prevention in patients with nonvalvular atrial fibrillation. We searched the MEDLINE database using the key words atrial fibrillation, thromboembolism, antithrombotic therapy, anticoagulation, stroke, warfarin, and aspirin.