During the past 2 decades, catheter ablation of atrial fibrillation (AF) has emerged as an important treatment option for patients with symptomatic AF refractory to antiarrhythmic drug therapy (1). Despite intense worldwide efforts to improve the safety and efficacy of this procedure, it is well recognized that success rates are modest, particularly when viewed from a long-term horizon (1–3). This is particularly true for patients with persistent AF (3). It is because of this limited long-term efficacy of AF ablation that considerable efforts are focused on new targets for AF ablation. These targets can broadly be grouped as ablation of the “fixed” substrate for AF and the “dynamic” substrate for AF. The purpose of this study was to provide new insights concerning the dynamic substrate for AF as assessed with the Focal Impulse and Rotor (FIRM) mapping system (Abbot Medical, Minneapolis, Minnesota), its relationship to the fixed substrate as assessed with echocardiography, and its potential impact on ablation strategies.