The large outcome studies of biventricular pacing to date have selected patients using electrocardiogram criteria (prolonged QRS and left bundle branch block morphology). However, 20-30% of patients do not appear to respond clinically, and as a result there has been much interest in developing more specific methods of detecting mechanical dyssynchrony. A number of different echocardiographic techniques have been developed which appear to offer greater sensitivity and specificity than ECG in selecting these patients. This paper reviews the most promising of the echocardiographic techniques and gives guidance for the clinical use of echocardiography in selecting patients for biventricular pacing.