Heart failure is one of the most prevalent diseases in industrialized countries. Although the prognosis of patients with heart failure is still poor, in recent decades new therapies have been investigated in order to improve quality of life and survival. However, up to 30% of the patients with advanced heart failure present disturbances in intraventricular conduction, and this produces asynchrony of ventricular contractility, leading to further deterioration in heart function. Cardiac resynchronization therapy can improve the synchrony of ventricular contractility. Numerous studies have demonstrated the benefits of biventricular stimulation therapy for improving hemodynamic parameters, quality of life, 6-minute walking test performance and functional class in patients with heart failure, ventricular systolic dysfunction and disturbances in intraventricular conduction. Some studies have demonstrated longer survival times in patients treated with cardiac resynchronization plus a defibrillator. Nonetheless, many questions about the benefits of heart resynchronization therapy, site of stimulation and best type of device (pacemaker or defibrillator) remain unresolved.