SMART-AV is a randomized, multi-center, double-blinded, non-significant risk, three-armed trial that will investigate the effects of optimizing AV delay timing in heart failure patients receiving CRT-D therapy. Patients will be randomized in a 1:1:1 ratio using randomly permuted blocks within each center. The study will compare chronic changes in structural and functional outcomes in CRT-D patients randomized to AV delay set as fixed, set with SmartDelay or determined by echocardiography. Patients will be followed at enrollment, implant, post-implant, 3-months post-implant and 6-months post-implant.