Electromechanical dyssynchrony is an important consequence of and contributor to ventricular dysfunction. Echocardiography can be useful to assess the mechanisms underlying mechanical dyssynchrony, to evaluate the impact of mechanical dyssynchrony on ventricular function, and to try to predict the therapeutic response to cardiac resynchronization therapy (CRT). Mechanical dyssynchrony has been demonstrated in several pediatric acquired and congenital cardiac conditions, but experience is still limited. Moreover, the optimal method to identify dyssynchrony remains unclear, and data predicting the response to CRT in pediatrics are lacking. Understanding mechanisms of electromechanical dyssynchrony by echocardiography seems promising, at least in left bundle branch block (LBBB), but may be limited in children due to the uncommon occurrence of LBBB in this population. This review addresses the commonly used methods to diagnose mechanical dyssynchrony, discusses the emerging concepts on the mechanisms of the various types of mechanical dyssynchrony, and discusses the possible significance of mechanical synchrony in pediatric and acquired congenital heart disease. (Echocardiography 2013;30:460-471).