Cardiac resynchronization therapy (CRT) is a device-based, nonpharmacologic approach that has shown to improve the outcome in patients with heart failure in terms of mortality and morbidity reduction. Large randomized trials have virtually enrolled patients in New York Heart Association class III-IV, with reduced left ventricular ejection fraction, with evidence of electrical dyssynchrony, and receiving optimal medical therapy and who were in sinus rhythm. Guidelines remain imprecise as to defining differentiated approaches according to the forms of atrial fibrillation other than permanent. These recommendations remain unsupported by evidence derived from randomized controlled trials, which are much needed.