The concept of Smart Buildings was introduced by the Energy Performance Building Directive, with the aim to promote energy flexibility, renewable energy production and user interaction. A wide range of definitions have been introduced in the literature to characterize smart buildings yet, at present, its’ concept and features are not clearly and uniquely defined. Simultaneously, building energy retrofit concept has been introduced to facilitate achieving the nearly Zero-Energy Building target and reduce energy consumption in existing buildings. Up to 90 % of the existing European building stock will still be standing and in use in 2050. Thus, there is a need to upgrade the existing retrofitting strategies into Smart Retrofitting, to achieve the nearly Zero Energy Building target and be able to respond to external dynamic conditions such as the weather and the grid. The aim of this research is first to review the concept of smartness in the built environment, highlighting the main features, functions, and technologies of smart buildings, also discussing the possible challenges for smart retrofit applications. The second part of the paper reviews the existing Key Performance Indicators that measure the performance and success in achieving goals in smart buildings. The need to develop a quantified guideline to improve energy and technological innovation is the basis for the increase of the smartness in buildings. Consequently, a set of nine groups of representative performance indicators for smart buildings is developed. This work shows current gaps in the literature and highlights the space for foreseeable future research.