Cardiac resynchronization therapy (CRT) is a promising technique in patients with end-stage heart failure. Current selection criteria include New York Heart Association class III or IV heart failure, left ventricular (LV) ejection fraction ≤35%, and wide QRS complex (>120 ms). The majority of patients selected according to these criteria respond well to CRT, but 20% to 30% do not respond. Selection criteria could be improved by including assessment of LV dyssynchrony, which appears mandatory for response to CRT. LV dyssynchrony may be evaluated using conventional echocardiography (eg, M-mode echocardiography), although tissue Doppler or strain imaging is preferred. Which technique will provide optimal information on LV dyssynchrony is currently unknown, as is the precise extent of LV dyssynchrony needed to predict response to CRT. Large multicenter studies are being performed to elucidate these issues...