The superbly refurbished and enlarged Wien Museum offers Vienna and its residents and visitors a modern, centrally located city museum. The decision to keep Wien Museum on Karlsplatz and to refurbish the historic structure led to a significant reduction in CO2 emissions in comparison with the realization of a new building. Further proof of the museum’s state-of-the-art sustainability is provided by its efficient energy management systems: from the use of geothermal and photovoltaic power to ensure an autarkic energy supply, via the green roof and the intelligent building services systems (such as the reuse of heat given off by visitors), to the clay wall cladding, which makes a natural contribution to maintaining a balanced indoor climate in the spaces of the permanent exhibition.