Atrial fibrillation (AF) is the most commonly encountered arrhythmia that occurs after cardiothoracic surgery. It develops in up to 40% of patients, most frequently in the first few postoperative days. The strongest risk factors for AF are age and male gender. The pathogenesis of this arrhythmia probably involves increased sympathetic activity in the setting of slowed atrial conduction. The development of postoperative AF is associated with an increased morbidity, higher costs, and prolonged hospitalization. The use of prophylactic beta-blockers and procainamide reduces the incidence of AF whereas digoxin, verapamil, and amiodarone appear less effective. Once AF occurs, the mainstay of treatment remains rate control, anticoagulation, and restoration of sinus rhythm.