Acute atrial fibrillation is the most common sustained, clinically significant dysrhythmia encountered in the emergency department (ED) and the most common dysrhythmia treated by emergency physicians. Atrial flutter is less common than atrial fibrillation but its management in the ED is very similar, and the majority of patients with atrial flutter also have atrial fibrillation. Symptomatic relief and ventricular rate control are generally the primary therapeutic objectives in the ED management of acute atrial fibrillation and flutter (AFF). The need for swift, appropriate action by the emergency physician is highlighted by the fact that up to 18% of patients with AFF develop potentially life-threatening complications such as congestive heart failure, hypotension, ventricular ectopy, respiratory failure, angina and myocardial infarction.