Although the estimates from limited studies vary on the proportion of patients with heart failure who also haveventricular dyssynchrony as reflected by a prolonged QRS complex, often in the form of left bundle branchblock, the number of such patients is large (27% to 53%) and it is certainly in excess of the rate for the generalpopulation. Among these patients, 10% to 15% are candidates for cardiac resynchronization therapy (CRT) viabiventricular pacing. Accumulated evidence from randomized controlled studies over the last few years hasindicated that significant hemodynamic and clinical improvement is conferred by CRT to the class III or IVheart failure patients with idiopathic or ischemic dilated cardiomyopathy and who also have a low left ventricularejection fraction (≤35%) and a wide QRS complex (≥120-150 ms). Newer data suggest a significant reductionin mortality and heart failure hospitalization, particularly when CRT is combined with an automatic defibrillatorbackup. This technique has transformed the traditional concepts associated with stimulation of the heart, and itis now being applied not only to restore an appropriate heart rate, but also to change the process of cardiacmechanical activation. Since this treatment must be integrated within a comprehensive and multidisciplinaryCHF 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 thesuccess rate of implanting left ventricular leads to >90%. Further confirmation from ongoing trials is eagerlyawaited, and more data from the studies on this procedure’s cost effectiveness are needed before CRT is consideredas a prime therapy in the heart failure population. (Korean Circulation J 2006;36:329-336)