We review the incidence of embolic events following cardioversion of atrial fibrillation, as well as the literature that forms the basis for the current strategy of anticoagulation before, and following, cardioversion to reduce the risk of post-cardioversion embolism. We evaluate a new strategy that uses transesophageal echocardiography to identify patients in atrial fibrillation without atrial thrombi who may be safely cardioverted without preceding anticoagulation and we also address the embolic event and anticoagulation issues in patients with atrial flutter. Cardioversion of atrial fibrillation to sinus rhythm is associated with a small but significant risk of thromboembolic events (average incidence, 1.5%; range, 0% to 7%). Anticoagulating these patients before cardioversion appears to significantly reduce this risk, and because of the delay in return of atrial contraction, anticoagulation should be continued for several weeks following cardioversion. The current guidelines for anticoagulating patients in atrial fibrillation who are to be cardioverted is based primarily on clinical observations, numerous uncontrolled case series, two retrospective trials, and one prospective nonrandomized controlled trial. Anticoagulation for 3 weeks before cardioversion followed by 4 weeks of anticoagulation after cardioversion is a theoretically sound and effective approach to reduce the risk of thromboembolic events. The use of transesophageal echocardiography to rule out thrombus and thus identify low-risk patients who may undergo cardioversion without preceding anticoagulation has been supported by several small studies that successfully used this strategy. However, the demonstration of a postcardioversion atrial and atrial appendage "stunning" suggests that anticoagulation needs to be given at the time of, and following, cardioversion. While promising, this transesophageal echocardiography--guided strategy for cardioversion of patients in atrial fibrillation requires more rigorous study before its routine use can be recommended. The current management of pure atrial flutter requires no anticoagulation before cardioversion; however, several clinical observation suggest theoretical risks for embolic events in these patients, thus further investigation of this strategy may be warranted.