Although the estimates from limited studies vary on the proportion of patients with heart failure who also have ventricular dyssynchrony as reflected by a prolonged QRS complex, often in the form of left bundle branch block, the number of such patients is large (27% to 53%) and it is certainly in excess of the rate for the general population. Among these patients, 10% to 15% are candidates for cardiac resynchronization therapy (CRT) via biventricular pacing. Accumulated evidence from randomized controlled studies over the last few years has indicated that significant hemodynamic and clinical improvement is conferred by CRT to the class III or IV heart failure patients with idiopathic or ischemic dilated cardiomyopathy and who also have a low left ventricular ejection fraction (< or =35%) and a wide QRS complex (> or =120-150 ms). Newer data suggest a significant reduction in mortality and heart failure hospitalization, particularly when CRT is combined with an automatic defibrillator backup. This technique has transformed the traditional concepts associated with stimulation of the heart, and it is now being applied not only to restore an appropriate heart rate, but also to change the process of cardiac mechanical activation. Since this treatment must be integrated within a comprehensive and multidisciplinary CHF management program, CRT has altered the medical practice of heart experts in the field of cardiac pacing. Technical advances with percutaneous methods that access the tributaries of the cardiac veins have raised the success rate of implanting left ventricular leads to >90%. Further confirmation from ongoing trials is eagerly awaited, and more data from the studies on this procedure's cost effectiveness are needed before CRT is considered as a prime therapy in the heart failure population.