European legislation requires that from 2020 all new buildings are nearly zero-energy buildings without proposing a concrete definition, leaving a number of uncertainties, specifically on the ambition level of such buildings in terms of energy consumption and CO2 emissions. This study analyzes key principles for a proper definition of nearly zero-energy buildings based on existing concepts across Europe and in line with the objectives and aims of the EU policy. This conference will take place at one of the largest passive office buildings in Brussels: AEROPOLIS II. The address: AEROPOLIS II, Avenue Urbain Britsiers 5, 1030 Brussels. Please note that the architects, engineers and owners of AEROPOLIS II will join us to confront the analysis with views and experiences of building professionals who have to turn theory into practice.