A significant fraction of the energy generated for industrial and domestic applications is lost in the form of heat. Because of this, thermal-energy storage materials are receiving increasing attention as a means of storing the generated heat for later use. In this paper, a brief description is given of two types of materials used in thermal-energy-storage devices – phase change materials for latent heat storage and photoswitches for chemical energy storage. In addition, we provide a succint account of the experimental and computational tools needed to understand the microscopic mechanisms of energy storage and to facilitate the rational design of new materials.