Cardiac resynchronization therapy (CRT) improves heart failure symptoms, cardiac function and long-term prognosis. As a result, it has been established as a treatment for refractory heart failure by using a specialized pacemaker to restore coordinated ventricular contractions with pacing. Despite being an invasive treatment, however, the above effects are not observed in 30-45% of patients selected based on the standard criteria that includes New York Heart Association class III or IV heart failure, left ventricular ejection fraction ≤35%, and QRS duration ≥120 or 130ms. From the fact that quantifiable resynchronization was associated with hemodynamic and clinical improvements, it should follow that mechanical dyssynchrony is a critical substrate for the benefits from CRT. The PROSPECT study unexpectedly demonstrated limitations of echocardiographic parameters using M-mode, pulsed-wave Doppler, and tissue Doppler imaging for accurately and reproducibly predicting response to CRT. However, advances in speckle tracking strain and real-time 3-D echocardiography have furthered the development of more sophisticated indices of dyssynchrony. Stress echocardiography might be useful for the detection of latent mechanical dyssynchrony in failing hearts. Because the substrate for CRT efficacy is multifactorial, a discriminant score that includes various clinical parameters and echocardiographic indices of mechanical dyssynchrony is needed to improve patient selection for CRT. (Circ J 2011; 75: 457-464)