In Europe, buildings operation causes 40% of energy consumption and 36% of greenhouse gas emissions. For this reason, the European Union’s Energy Performance of Building Directives (EPBD) provides policies aimed at optimizing the energy efficiency in buildings. To achieve the goal of optimizing the energy performance of buildings, the directive identifies several technological solutions that can be used. One of these technological solutions and the topic of this thesis are the Building Automation and Control Systems (BACS), whose purpose is to adapt the operation of technical building systems on the real behavior of users according to the external environmental conditions. To develop this work the EN ISO 52120 standard has a relevant importance, since it provides the list of BAC systems, the method to define the BACS efficiency classes, and the detailed and simplified methods to assess the effect of BACS on the building energy performance.