Atrial fibrillation is the most common arrhythmia in clinical practice, may coexist with conditions common to both cardiovascular and noncardiovascular diseases and is associated with considerable morbidity and mortality. Atrial fibrillation is often asymptomatic and diagnosed only when it has caused a potentially serious complication, such as an ischemic stroke. When atrial fibrillation has been identified, 2 objectives have to be addressed--the antiarrhythmic therapy based on rate control or rhythm control, and prevention of thromboembolism. A rhythm or rate control strategy can be chosen indifferently because they have comparable efficacy for the outcome measure of mortality, but the antithrombotic therapy is ever mandatory. The risk of stroke increases cumulatively with increasing age, previous transient ischemic attack or stroke, hypertension, diabetes mellitus, impaired left ventricular function and heart failure. Warfarin reduces the risk of stroke by about two thirds; and aspirin, by about one fifth, but its use must be weighted with the risk of bleeding. The risk of anticoagulant-associated hemorrhage increases with age, the presence of serious concomitant diseases, with poorly controlled hypertension and poorly controlled anticoagulation.