This article discusses how echocardiography can be applied to quantify dyssynchrony in patients who are evaluated for cardiac resynchronization therapy (CRT). A number of echocardiographic indices have been proposed as markers of success of CRT. However, when tested against QRS width in prospective clinical trials, none of the echocardiographic indices are proven to give clinical benefit. One important message in this review is that future studies should focus on approaches which can differentiate between electrical and non-electrical aetiologies of dyssynchrony, since only electrical dyssynchrony is likely to respond to CRT. Just measuring velocity indices does not identify the aetiology. Myocardial strain appears more promising, but one should be aware that timing of peak systolic strain is determined not only by electrical conduction. It is proposed to use onset septal shortening during pre-ejection for timing of earliest left ventricular (LV) electrical activation. One should take into account potential ischaemia, scarring, and other structural changes as contributors to dyssynchrony. As a method to identify electrical dyssynchrony, the authors propose to use time of active force generation as defined by LV pressure-strain loops. A non-invasive method to measure segmental pressure-strain loops is also proposed as a means to quantify the impact of dyssynchrony on distribution of myocardial work. Furthermore, it is important to be aware that LV dyssynchrony may have a combination of aetiologies, not all amenable for CRT.