Patients with heart failure, marked systolic dysfunction and dys-synchronous myocardial contraction are at increased risk for exacerbated pump failure and arrhythmias, and suffer higher mortality rates. In these patients, cardiac resynchronization therapy (CRT), achieved by a biventricular pacing, can lead to clinical improvement, reverse left ventricular remodeling and prolonged survival, although a substantial subset of non-responders has been reported. The electrocardiographic QRS width does not predict long-term CRT benefit, while direct measures of mechanical dyssynchrony obtained by various ultrasound techniques have been shown to have a better predictive value. However, currently there is no consensus on the optimal ultrasound technique to use for assessing mechanical dyssynchrony and select candidates for CRT. In this review, the clinical role of both conventional and new ultrasound techniques applied to the study of cardiac dyssynchrony will be discussed, focusing on patient selection for CRT.