Evidence is emerging to indicate that atrial fibrillation (AF) is independently associated with an increased risk of sudden cardiac death (SCD). This association has been consistently observed in specific patient subgroups such as patients with myocardial infarction (MI), heart failure, and hypertension, and importantly, in the general population. Data from studies of implantable cardioverter-defibrillator recipients suggest that the rapid and irregular rhythm of AF and the short-long-short cycles that are highly prevalent in AF increase susceptibility to ventricular tachycardia and ventricular fibrillation. An alternative explanation for the association between AF and SCD includes confounding or mediation by shared risk factors such as coronary artery disease and heart failure. Possible risk factors for SCD in patients with AF include black race, left ventricular hypertrophy, history of MI, and diabetes. Additional research is needed to confirm the inherent proarrhythmic nature of AF, identify patients' characteristics or clinical conditions that potentiate SCD risk, and define effective SCD prevention strategies for patients with AF. (Circ J 2014; 78: 2588-2593).