Energy efficiency in buildings is dependent on many different factors; the material quality of the building, and the various technologies implemented. Nonetheless, the use and the users of buildings are also of great significance. This paper analyses building operators and their role in making buildings “work”. Building operators may be viewed as an important user group in buildings. In contrast with “end-users”, a group that has been much studied over the last years, building operators have special competence in operating the building and thus also have a better opportunity to influence the innovation process. The paper argues that the building operators may be seen as important actors in optimizing energy efficiency in buildings. Furthermore, it argues that they hold tacit and experience based knowledge that could be utilised to a greater extent in the design and construction phase and not only in the operation phase. Building operators negotiate between users, technologies and the building, and thus, their understanding of and knowledge about the building is vital for reaching goals of energy efficiency.