Cardiac resynchronization therapy (CRT) is used to treat a subset of heart failure patients with discoordinate wall motion. Defining appropriate patients is important, and, although electrical delay (wide QRS) is commonly used, recent data show that measures of mechanical dyssynchrony improve the sensitivity and specificity of predicting responders (including patients with narrow QRS complexes). This has stimulated studies of dyssynchrony per se, and the phenomenon now appears to be very common in virtually all forms of heart failure. However, what all this dyssynchrony means clinically, and how or whether it should be treated by CRT or other means, remains unclear.