The 1st International Conference on Buildings, Energy, Systems, and Technology (BEST 2016), was held in Belgrade, Serbia, on November 2-4,2016. This conference was an opportunity to bring together experts with different skills around the theme of building performance. It covered different aspects (technology, energy, environment, and economy), different scales (from the urban to the building level, including the built environment up to the single technical building systems), and different phases of the building process (from urban planning to building design, up to construction and management). This Special issue contains extended versions of some selected papers presented at BEST 2016, which went through a blind peer review of international experts. The present Editorial introduces the topics of the BEST 2016 and the contents of this Special issue. The focus of the Editorial is the theme of energy efficiency improvement in buildings; it provides an overview of state-of-the-art and of the legislation framework, and a discussion on research perspectives and trends.