Cardiac resynchronisation therapy (CRT) has emerged as a treatment option for patients with severe, drug-refractory heart failure and signs of intraventricular dyssynchrony. In clinical trials CRT reduced the overall mortality, improved symptoms, exercise tolerance, and left ventricular function, as compared with optimised medical therapy alone. One of the challenging fields in patient selection for CRT is to identify the 20-30% of heart failure patients with bundle branch block that will not respond to this novel therapy. Other fields of uncertainty, such as CRT in patients with atrial fibrillation or chronic right ventricular stimulation as well as the role of a back-up defibrillator will be discussed.