Episodes of sustained atrial fibrillation have long been effectively treated with external, transthoracic defibrillation. Despite concomitant, postcardioversion therapy with antiarrhythmic agents, patients will frequently have additional episodes of atrial fibrillation requiring either repeat external cardioversion or treatment with either pharmacologic or additional nonpharmacologic therapies. The limited long-term efficacy of different treatment regimens has resulted in the development and evaluation of newer, nonpharmacologic therapeutic options including the implantable atrial defibrillator. The feasibility of an implantable atrial defibrillator has been supported by recent advances in the areas of implantable ventricular defibrillators, including device size and transvenous lead systems. The concept is also supported by the demonstration in recent studies that transvenous, internal cardioversion of atrial fibrillation is possible using low energy shocks. Preliminary data suggest that low-energy atrial defibrillation shocks synchronized to an R wave with a relatively long coupling interval does not result in potentially lethal ventricular arrhythmias. Patient tolerance, and therefore acceptance of therapy, is presently a topic of significant concern and evaluation. Although present data suggest that the concept of an implantable atrial defibrillator is a potential treatment option for recurrent, persistent atrial fibrillation, significant clinical evaluation is required to define the patient population and overall clinical use of this type of device, either as a stand-alone therapy or in combination with other pharmacologic and nonpharmacologic therapies.