Atrial fibrillation (AF) is an abnormal rhythm characterized by chaotic atrial electrical activity resulting in loss of atrial contraction, an irregular and unpredictable heart rate, and a tendency for thrombus formation. The prevalence of AF is estimated at 1 to 2%, but it’s likely higher than that because one-third of patients may have no symptoms and might never seek medical attention. Data suggest that 1 in 4 people over the age of 40 will develop AF in their lifetime. About 10% of patients over age 80 have experienced the arrhythmia, and some estimates predict the prevalence will double in the next 50 years. This chapter discusses the pathophysiology, genetics, diagnosis, classification, and treatment of AF. Figures show atrial fibrillation and coarse atrial fibrillation plus common right atrial flutter. One algorithm is for oral anticoagulation therapy, and a second shows a recommended hierarchical choice of antiarrhythmic therapies versus catheter ablation for recurrent symptomatic atrial fibrillation. Tables list classification, diagnostic evaluation of, clinical consequences of, and conditions often associated with atrial fibrillation. Three scoring systems are included: 1) for congestive heart failure, hypertension, diabetes, stroke, and transient ischemic attack; 2) to assess the risk of bleeding with oral anticoagulation, and 3) data and proportion of patients from the Euro Heart Survey. Other tables include long-term anticoagulation guidelines for atrial fibrillation, intravenous drugs used for acute rate control, oral drugs used for chronic rate control, and antiarrhythmic drugs for conversion of atrial fibrillation and/or maintenance of sinus rhythm. In addition, there’s a summary of randomized trials weighing rate control and rhythm control strategies, plus schemes for categorizing thromboembolism risk. This review contains 4 highly rendered figures, 13 tables, and 129 references.