Cardiac resynchronization in heart failure already has a history of 12 years. However, the major advances have been the result of large multi center trials dating from 2001. In all these trials patients with a LVEF < or = 35% were included, and a QRS above 120 msec. Follow up was from 3-36 months. The majority of these trials showed a positive effect in reduction of composite and points of death or hospitalization for major cardiovascular events. Many of these trials also showed a diminution of left ventricular and systolic diameter or volume. Even in NYHA class II patients an improvement was seen. Some unanswered questions still remain as regards the agreement on electrical or electromechanical dyssynchrony criteria. There is a number of patients with "wide" QRS who do not improve and conversely a number of patients with a narrow QRS who witness improvement. The benefit in patients with atrial fibrillation also remains unanswered. Finally the value of this modality in patients with mild heart failure or asymptomatic left ventricular systolic dysfunction, NYHA class I-II remains to be determined in large on going trials. Another question is whether biventricular or left ventricular patient is preferable. Finally whether biventricular patient should be complemented by a defibrillator insertion is being currently studied. Cardiac resynchronization therapy along or in combination with an ICD improves symptoms, reduces major morbidity and mortality in patients with a left ventricular EF<35%, ventricular dilatation and a QRS > or = 120 msec in NYHA class III-IV. Further indications are currently being examined.