Energy consumed by heating, ventilation and air conditioning systems in buildings represents an important part of the global energy consumed in Europe. Thermal energy storage is considered as a promising technology to improve the energy efficiency of these systems, and if incorporated in the building envelope the energy demand can be reduced. Many studies are on applications of thermal energy storage in buildings, but few consider their integration in the building. The inclusion of thermal storage in a functional and constructive way could promote these systems in the commercial and residential building sector, as well as providing user-friendly tools to architects and engineers in order to help implementation at the design stage. Thermally activated building systems (heating and cooling systems integrated in the building structure) are a technology with potential for significantly reducing buildings energy use. They mainly exchange heat through radiation and are able to store heat in the building’s thermal mass but the interaction of these systems with the building structure make their energy evaluation and design process difficult. A lot of research is conducted to develop control strategies that fully exploit its energy saving potential and maximize the use of renewable energies.