This chapter introduces the role of thermal energy storage (TES) systems in enhancing building energy flexibility and demand-side management. Different forms of TES and their related storage media and materials were reviewed and the link between TES systems and energy flexibility was discussed. The role of TES systems in building energy flexibility and demand-side management was then addressed in terms of long-term (seasonal) and short-term storage applications. Different TES materials or media, such as sensible heat storage by water tanks, aquifers, and latent heat storage by various phase change materials can be considered. Furthermore, different techniques that can be utilized to manage the demand with the aid of TES systems are discussed. Different approaches, such as model predictive control as one of the most prominent ones, can be considered to maximize the benefit of TES in demand-side management, and hence energy flexibility.