Despite continuous progress in drug therapy, many patients are still progressing into advanced heart failure, a very poor condition in terms of quality of life and prognosis. Therapeutic resources are quite limited at that end-stage. Heart transplantation may only be proposed to a small minority of patients. New non-pharmacological alternatives like cellular cardiomyo-plasty or left ventricular (LV) implantable assist device are still under evaluation. So simpler and cheaper approaches have imperatively to be developed for treating this highly invalidated and rapidly growing and ageing population. Cardiac resynchronization therapy (CRT) with multisite biventricular pacing has been initiated in France in early 90's. The aim of CRT is to try and correct the electromechanical abnormalities that result from antrioventricular and intraventricular conduction delay (IVCD), a very common observation (30-50%) in patients with chronic heart failure (CHF). IVCD worsens progressively overtime and is responsible for discoordinated interventricular and left-intraventricular contraction-relaxation which in turn enhances the hemodynamic consequences of the baseline LV systolic dysfunction. This new therapeutic concept was first assessed in acute hemodynamic studies with temporary pacing, then in pilot studies with permanent transvenous biventricular pacing. Several controlled studies (MUSTIC, MIRACLE, CONTAKCD...) were conducted afterwards and demonstrated that CRT might improve significantly symptoms, quality of life and exercise tolerance in patients with severe heart failure (NYHA class III -- IV) under optimized drug treatment, low ejection fraction and significant IVCD as indicated by an intrinsic QRS duration QRS> 150 MS. The rehospitalization rate was also significantly reduced with CRT. The clinical benefit was preserved over at least 1 year follow-up. The first validation stem is now completed. There are however several important questions yet to be answered. Which is the impact of CRT on all-cause mortality and sudden cardiac death? Two large-scale studies, CARE-HF in Europe and COMPANION in USA, are ongoing to try and answer this question. Which type of implantable devices has to be developed preferentially: multisite pacemakers or multisite pacemaker-defibrillators? May CRT induce LV reverse remodelling and thus help at preventing heart failure progression? Which cost-effectiveness ratio for heart faulure management? How to better select potential responders?... The whole validation process of CRT should be completed on 2004-2005.