Cardiac resynchronization therapy (CRT) has shown improvement in mortality and morbidity in patients with left ventricular systolic dysfunction and congestive heart failure. Additionally in CRT patients it has been demonstrated that optimizing paced/sensed atrioventricular (AV) and interventricular (V-V) timings leads to immediate hemodynamic benefits and further improves cardiac function. Recent studies have shown that optimal paced/sensed AV and V-V delays change over time, which raises the question of how often optimization should be repeated. Thus, frequent re-optimization of these delays might be beneficial for maintaining significant improvement of cardiac function. However, it remains to be evaluated whether timing optimization may be beneficial on patients who have received CRT for a number of years and are now having the CRT device replaced.