Atrial fibrillation (AF) is a common acute or chronic cardiac disorder that can result in significant morbidity and mortality. Its incidence in the United States is increasing. Projections suggest that more than 5.6 million Americans (50% of whom will be > or =80 years of age) will have AF by 2050. The American College of Cardiology, American Heart Association, and the European Society of Cardiology define AF as a supraventricular tachyarrhythmia characterized by uncoordinated atrial activation with consequent deterioration of atrial mechanical function. On an electrocardiogram, AF is characterized by the replacement of P waves by rapid oscillations or fibrillatory waves that vary in size, shape, and timing. Evidence suggests that histological changes exist in the atria of patients with AF, however, it is not known if these changes are a cause or a consequence of AF. Although the fundamental mechanism underlying the disorder is not known, clinical identifying factors are associated with the condition. These may be divided into noncardiac (thyrotoxicosis, alcohol use, electrolyte imbalance, certain pharmacologic and recreational drugs) and cardiac causes (any cause of enlarged left atrium, poor ventricular function, heart surgery). The principles of treatment for this condition are to stabilize the patient hemodynamically, simultaneously determine whether a reversible cause of the AF exists, control the patient's heart rate, determine whether the patient should be cardioverted or maintained in AF, and then develop strategies to prevent the most important complications of stroke. This article will describe in detail the acute management of AF as well as its epidemiology.