Heart failure is a common disease, affecting 2-3% of the population in the western world. About 30% of patients with heart failure and reduced ejection fraction display signs of electrical dyssynchrony on ECG, usually left bundle branch block (LBBB), which is associated with a worse prognosis. Cardiac resynchronization therapy (CRT) reduce mortality for patients with dyssynchronic heart failure, defined as ejection fraction (EF) = or \< 35% and LBBB. About 1/3 of the patients that fit CRT criteria will not respond to CRT. Which patients that will turn out to be non-responders cannot be anticipated beforehand. We have started a clinical study to collect blood samples, heart tissue and clinical data from heart failure patients eligible for CRT and a control group of heart failure patients on medical therapy. Patients will be assessed before CRT implantation or early after initiation of medical treatment, at 3 months, 6 months and 12 months. Blood samples and tissue will be analysed in the search for (i) biomarkers to separate CRT responders from non-responders and (ii) mechanisms behind the remodelling observed in CRT and with medical therapy.