Cardiac resynchronization therapy (CRT) is a new therapeutic approach for a selected group of patients with symptomatic heart failure (NYHA functional class III-IV) despite optimal medical therapy, due to dilated cardiomyopathy of any etiology (left ventricular ejection fraction < or = 35% and left ventricular end-diastolic diameter > or = 55 mm), who present with electromechanical dyssynchrony (QRS > or = 130 ms). Safety and effectiveness of CRT have been demonstrated by several clinical trials, with patients achieving significant improvement in both clinical symptoms as well as functional status and exercise capacity. Furthermore, CRT has reduced morbidity of heart failure patients, while its impact in improving survival still remains to be clarified. Whether or not heart failure patients candidate to CRT should receive a defibrillator back-up remains debatable, although growing evidence is pointing to extensive use of a defibrillator in such a population.