Atrial fibrillation—a major health problem, particularly among the elderly—is associated with an increased risk of stroke, death, and heart failure. Yet, despite more than 120 years of research, the detailed molecular, cellular, and pathophysiological mechanisms remain poorly understood. The major recent advance in the management of these patients has come from clinical trials, which have identified age >65 y, history of hypertension, stroke, diabetes, and poor ventricular function as factors that independently predict an increased risk of stroke. Thus, patients >75 y with or without risk factors or patients <75 y with a single risk factor derive benefit from anticoagulation with warfarin, with good blood pressure control. The remaining patients have a low risk of stroke on aspirin, 325 mg/day. Consideration should always be given to cardioversion and maintaining sinus rhythm...