Ventilation systems should contribute to good indoor air quality conditions and should be energy efficient. In practice, one often finds ventilation systems which often do not give the appropriate indoor climate conditions and/or which consume a lot of energy.""In an increased number of European countries, legislation exists (France, Netherlands,...) or is under preparation (Belgium, Greece,...) putting requirements on the total energy use of the building. Part of this energy use deals with ventilation (thermal energy and fan energy). Various aspects of the notion of quality are discussed and a number of challenges for such energy performance legislation are identified.