Cardiac resynchronization therapy (CRT) is a nonpharmacological method used to treat chronic heart failure. One symptom of heart failure is ventricular dyssynchrony decreasing the stroke volume. CRT enables treatment of ventricular dyssynchrony by means of delivering pacing pulses to the right and left ventricle. As a consequence, a better heart contraction mechanism and an increase in cardiac output is achieved. The clinical efficiency of CRT was confirmed in large randomized studies, which are described in the Chap. 6, dealing with indications for treatment. CRT improves hemodynamic parameters and quality of life of patients with severe heart failure. The first CRT systems made use of a left ventricular (LV) lead inserted via an epicardial approach requiring thoracotomy. Today, the lead is inserted via the subclavian vein through the coronary sinus and into the target coronary vein. The pacing electrodes of the LV lead should be suitably located at the place of the most recent ventricular activation during intrinsic cardiac impulse conduction. In most patients, this site is assumed to be on the left ventricle lateral wall if the lateral or posterolateral vein is available.