This paper deals with the methods and applications of describing and assessing thermal energy storage (TES) systems in buildings. Various technical aspects and criteria for thermal energy storage systems and applications are discussed and energy saving techniques and environmental impacts of these systems are highlighted with illustrative examples. An energy and exergy analysis of thermal energy storage systems and their sub-processes is presented for system design and optimization. An illustrative example is also given to demonstrate how exergy analysis provides a more realistic and meaningful assessment than the conventional energy analysis of the efficiency and performance of a thermal energy storage system.