This chapter gives an overview of energy storage systems, focusing on thermal energy storage (TES) as a key technology for addressing the timing gaps between energy supply and demand. It explains the principles and types of TES, including active and passive systems, and highlights their roles in improving energy efficiency and cutting emissions. The chapter divides thermal storage methods into sensible, latent, and thermochemical systems, detailing their mechanisms, materials, and relative performance. It discusses storage media such as liquids, solids, salts, and metals, with special attention to phase change materials (PCMs) for latent heat storage. It examines strategies for improving heat transfer, such as encapsulation, conductive structures, and composite materials, for both passive and active setups. It also covers integration methods for industrial and solar uses, including high-temperature steam and thermocline systems. In the end, thermochemical storage is identified as a promising method for long-term and high-density energy storage, offering paths toward sustainable and efficient thermal management.