Notions about the mechanisms underlying atrial fibrillation (AF) go back to the beginning of the twentieth century, when multiple circuit reentry, rapidly discharging ectopic foci, and single-circuit mother-wave reentry with fibrillatory conduction were proposed as alternative hypotheses to account for the characteristic rapid, irregular atrial activity. For many years, multiple-circuit reentry held sway as the dominant conceptual model of AF. Recent research has shown that all of the initially-proposed mechanisms can play a role in AF, depending on the clinical setting, and that atrial tachycardias (epitomized by AF) alter atrial electrophysiology so as to favor multiplecircuit reentry, which then becomes a common final pathway for AF. Recent advances in understanding the mechanisms of AF have important implications for treating the arrhythmia, which the present paper will review...