Seen from the AIVC Technotes 21 and 28, Ventilation Efficiency is still a complex concept. As well for measurements as for simulations. Two more or less separate terms are used: Ventilation Efficiency (-Supply Efficiency) and Ventilation Effectiveness (-Contaminant Removal Effectiveness). In thispaper is shown that the Multizone Ventilation Efficiency has a much wider range than Ventilation Efficiency within one room. In a single room efficiencies can be found for example up to 2 forvery good systems.