Patients with moderate or severe heart failure often have some form of intraventricular conduction abnormality and increased QRS duration on the routine electrocardiogram. The most common pattern is the left bundle branch block, when the electrical activation of both ventricles is disturbed and the lateral wall of the left ventricle is significantly delayed. The use of cardiac pacing to coordinate the impaired electrical activation and myocardial contraction is called cardiac resynchronization therapy (CRT). Randomized trials of cardiac resynchronization were demonstrated to improve left ventricular systolic function, exercise tolerance, quality of life, and reduction in rehospitalization frequency of the patients. Resynchronization also prolongs survival in patients with NYHA Class III or IV heart failure and left ventricular ejection fraction ≤35%. Recent developments using electro-anatomic mapping, contact and noncontact endocardial mapping have demonstrated that the correct positioning of the pacing electrodes provides better resynchronization and better response to CRT. Body surface potential mapping and noninvasive electrocardiographic imaging provide also a deeper insight into the mechanism of cardiac electrical depolarization and contributing to develop the selection method of best pacing sites for patients referred for CRT.