Atrial fibrillation (AF) is the most common sustained arrhythmia, affecting an estimated 2.2 million adults in the United States. The median age of people with AF is 75, and it affects 8.8% of the US population > 80 years of age. Prevalence data from other countries are presented. Direct comparisons are limited by study design, but rough comparisons suggest that the prevalence of AF in Europe is similar to the prevalence in the United States, whereas the prevalence in Asia may be lower. The limited comparative data underscore our lack of understanding of AF risk factors and complications in racial subgroups and in developing countries. AF increases stroke risk 5-fold. The clinical features that predict higher risk of stroke in AF are prior stroke, hypertension, advancing age, diabetes, and congestive heart failure. Predicting which patients with atrial fibrillation are at the highest risk of stroke remains a challenge. Echocardiographic findings have been investigated to assist in the risk stratification of patients with AF. Despite evidence from clinical trials that anticoagulation with warfarin reduces stroke incidence and even mortality, anticoagulation remains underutilized, especially in the elderly. Improvement in the rate of anticoagulation in patients with AF at risk of stroke can be expected to decrease the complications and mortality of AF.