Thermal energy storage (TES) is the capture and storage of thermal energy (heat or cold) for use at a later time. Applications span grid-scale (storing solar heat for night power generation), industrial (capturing waste heat), and building-level (storing heating/cooling). TES is critical for decarbonizing energy systems where renewables (solar, wind) are intermittent. TES systems include sensible heat storage (heating water, rocks, salt), latent heat storage (phase-change materials), and thermochemical storage (reversible chemical reactions). Each has different trade-offs: sensible is simple but bulky; latent is compact but expensive; thermochemical has highest energy density but complex.