Patients with dilated cardiomyopathy and discoordinate wall motion due to intraventricular conduction delay are at increased risk for exacerbated pump failure and arrhythmias and suffer higher mortality rates. Biventricular and left ventricular resynchronization pacing therapies acutely improve systolic ventricular function and energetic efficiency in patients with heart failure and left-bundle–type intraventricular conduction delay. Sustained therapy can further inhibit or reverse chronic chamber dilation and remodeling. As with all therapies for heart failure, individual subject responses are variable; however, the invasive nature and expense of resynchronization therapy has particularly highlighted the need to prospectively identify optimal candidates. Although QRS duration has been principally used to date, increasing evidence shows this to have poor acute and chronic correlations with patient response. In contrast, direct measures of mechanical dyssynchrony based on simple echo imaging and more complex tissue Doppler and magnetic resonance imaging–based approaches appear to afford better predictive accuracy.