Biventricular pacing is a new development in the treatment of ventricular failure associated with intraventricular conduction delays, and the term «ventricular resynchronization» has been coined, implying for many authors that synchronous right and left ventricular activation and contraction are the goal of therapy. However, there is ample evidence that isolated left ventricular stimulation may be at least as efficacious as biventricular stimulation, and the mechanisms of functional improvement remain speculative. The role of mitral regurgitation and its modification with «resynchronization» has not been fully evaluated. Long-term prognosis, effect on mortality and predictors of a positive response are important unanswered questions. It is clear that a narrow-based QRS complex is not a good indicator of a favorable response. We need to better understand the effect of the activation sequence on left ventricular contraction dynamics, including mitral valve function, to refine the technique and indications for «resynchronization therapy».