The research field of the chair offers new opportunities for energy policy assessment and for the identification of efficient measures and systems for buildings in use. This is relevant to two new developments. On the one hand, the complexity of buildings’ energy systems is increasing a lot, as a consequence of multiple integrated conversion, distribution, and buffering systems inherent to the use of renewable thermal or electrical energy. Unlike industrial systems, buildings’ energy systems work in a strong interaction with the building itself and its occupants, being responsible for the quality of the indoor climate. This complexity often leads to disappointing operational performances.