Atrial fibrillation (AF) is often complicated by a life-threatening ventricular response, and emergency electrocardioversion and/or drug therapy to reduce the rapid ventricular rate may be necessary. However, patients with AF and Wolff-Parkinson-White syndrome should not be given digoxin or calcium channel blockers. Elective direct current (DC) cardioversion has a higher success rate and a lower incidence of cardiac adverse effects than does medical cardioversion in converting AF to sinus rhythm. Unless transesophageal echocardiography has shown no thrombus in the left atrial appendage before cardioversion, oral warfarin should be given for 3 weeks before elective direct current or drug cardioversion of AF and continued for at least 4 weeks after maintenance of sinus rhythm. Many cardiologists prefer, especially in older patients, ventricular rate control plus warfarin rather than maintaining sinus rhythm with antiarrhythmic drugs. Patients with chronic or paroxysmal AF at high risk for stroke should be treated with long-term warfarin to achieve an international normalized ratio of 2.0-3.0. Patients with AF at low risk for stroke or with contraindications to warfarin should be treated with aspirin 325 mg daily. Management of atrial flutter is similar to management of AF.