The role of echocardiography in cardiac resynchronization therapy (CRT) may be considered under the three sections following as patient selection for CRT, targeting lead placement in CRT, and optimizing device programming postimplant. This chapter covers the role of echocardiography in patient selection for cardiac resynchronization therapy, echocardiographic techniques used to select patients for CRT, role of echocardiography in targeting lead placement for CRT, and role of echocardiography in optimizing CRT programming postimplant. Echocardiography is most commonly used in the postimplant setting, both acutely and in the long-term, to optimize CRT settings by adjusting the programmed AV and VV delays to achieve maximum response to CRT. AV delay optimization refers to programming the atrioventricular pacing delay in the CRT to achieve the optimum PR interval that will maximize diastolic filling without elevating the atrial pressures.