Heart failure is a leading cause of morbidity and mortality. Cardiac resynchronization therapy (CRT) is a well-established treatment for patients with symptomatic heart failure in spite of optimised medical treatment (OMT), reduced left ventricular pump function with left ventricular ejection fraction (LVEF) ≤ 35% and prolonged activation of the ventricles (bundle branch block: BBB). CRT is established by implanting an advanced pacemaker system with three leads in the right atrium, right ventricle, and in the coronary sinus (CS) for pacing the left ventricle (LV), and often is combined with an implantable defibrillator (ICD) function. On average, CRT treatment improves longevity, quality of life and functional class, and reduces heart failure symptoms. Thus, at present, CRT is indicated for heart failure patients on OMT with BBB or chronic right ventricular (RV) pacing. It is, however, a significant problem that 30-40% of CRT patients do not benefit measurably - showing symptomatic improvement or improved cardiac pump function - from this therapy (socalled non-responders). LV lead placement is one of the major determinants of beneficial effect from CRT. Observational studies and three randomised trials with small sample sizes indicate that targeted placement of the LV lead towards a late activated segment of the LV may be associated with improved outcome. Based on this literature, some physicians already search for late activation when positioning the LV lead. However, such a strategy was never tested in a controlled trial with a sample size sufficient to investigate important clinical outcomes. Detailed mapping for a late activation may increase operating times and infection risk, result in use of more electrodes and wires, thereby increasing costs, and increase radiation exposure for patient and staff. Placement of the LV lead in late activated areas close to myocardial scar may even result in higher risk of arrhythmia and death. At present, it is completely unsettled whether targeted positioning of the LV lead to the latest electrically activated area of LV is superior to contemporary standard CRT with regard to improving prognosis for patients with heart failure and BBB. The present study aims to test whether targeting the placement of the LV lead towards the latest electrically activated segment in the coronary sinus branches improves outcome as compared with standard LV lead implant in a patient population with heart failure and CRT indication.