Cardiac resynchronization therapy (CRT), or biventricular pacing therapy, is the first approved nonsurgical treatment of advanced heart failure (HF) employing implantable device. This therapy is characterized by the implantation of an additional left ventricular (LV) lead, usually through the coronary sinus, to reach the lateral or postero-lateral vein in order to pace the LV free wall. The conventional right ventricular lead is usually needed to pace the septal region while the right atrial lead is placed to provide sensing and back-up pacing when necessary. CRT can be delivered by implantation of a pacemaker (CRT-P) or a defibrillator (CRT-D). In HF population, about one-quarter will exhibit features of prolongation of QRS duration, which is also a marker of poor prognosis. Because of the presence of electrical activation delay within the LV, these patients will develop electromechanical delay in the form of systolic dyssynchrony (i.e., uncoordinated contraction in different regions of the LV). This will accelerate the pathophysiologic process of LV adverse remodeling with cavity dilatation, deterioration of systolic function, elevation of filling pressure and mitral regurgitation. The current guidelines recommend CRT for HF patients with New York Heart Association (NYHA) class III or IV symptoms despite optimal medical therapy, reduced ejection fraction of 120 or 130 ms.