On the basis of a representative set of examples, we tackle following issues concerning the energy performance gap which is commonly observed in retrofit of multifamily buildings: quantification of the main factors behind the gap, in relation with simulation; decomposition of the gap, by combination of the most sensitive parameters, in relation with observed values; exploration of how to bridge the gap, on the basis of two best practice examples. The results show that the gap consists of: (i) the difference between optimal yet realistic conditions of use, and standard values used in the simulation process; (ii) the difference between actual and optimal conditions of use, which corresponds to the optimization potential. It proves that significant reduction of the performance gap is possible through building optimization and responsible behavior.