Cardiac resynchronization therapy is an important and underused tool to help patients with heart failure symptoms, left ventricular systolic dysfunction (LVEF≤35%), and intraventricular conduction system disease (QRS≥120 msec). Cardiac resynchronization therapy paces the heart simultaneously from both right and left ventricles (through the coronary sinus). Approximately three quarters of patients who undergo a successful implant will have some degree of symptomatic improvement and have fewer heart failure hospitalizations. When cardiac resynchronization therapy is combined with a defibrillator, patients may benefit from the added protection against sudden arrhythmic death.