Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia and is a frequent complication of acute myocardial infarction (MI). AF occurs in 5% to 10% of patients who have received fibrinolysis. Post-MI AF is more common in older patients, in patients with heart failure, and after more extensive infarction. Postinfarction prognosis is worse among patients complicated by AF--indeed, mortality and morbid events including stroke, thromboembolism, and heart failure are increased in this group. It is unclear as to whether AF directly reduces survival or merely demarcates patients at higher overall risk. Despite its frequent occurrence and deleterious influence on outcomes, randomized data regarding management of AF after acute MI are scarce. This review summarizes recent data charting the incidence of AF after acute MI and describes features associated with its occurrence. Clinical sequelae and current principles in treatment are also discussed.