Cardiac resynchronization therapy (CRT) has been shown to improve mortality and morbidity in adults with refractory heart failure and prolonged QRS-duration. Recent research data suggest that the therapeutic benefit is related to the effect of CRT on interventricular and intraventricular dyssynchrony associated with electrical dyssynchrony. However, around 30-40% of the patients do not respond to CRT when device implantation is based only on QRS-duration. It was hoped that improved description of mechanical dyssynchrony using imaging techniques, might result in improved identification of patients who could benefit from CRT. Different methods have been proposed but a recent multicenter prospective echocardiographic study (PROSPECT) was disappointing. Applying adult criteria for CRT treatment to children and adults with acquired and congenital heart disease is even more challenging due to the age-dependency of QRS-duration and the wide variety of underlying diseases including different ventricular morphology that can result in heart failure. In this review we will overview the adult and pediatric data of CRT treatment and propose a mechanistic approach that could potentially be helpful in trying to identify those patients who might benefit from the treatment.