In this chapter, direct storage of heat in insulated solids or fluids is possible even at comparatively low temperatures (theoretically from t>0°C), but energy can only be recovered effectively as heat. Hot rocks and fireplace bricks have served as primitive heat storage devices from ancient times. This is still the case in industrial furnaces and in the baker's electric oven, where cheap electricity is used to heat the oven during the night. High temperature thermal storage can be used both to utilise heat in industrial processes and for heat engines. One recent example is the power supply for Stirling engines. Thermal energy storage (TES) is ideally suited for applications such as space heating, where low quality, low temperature energy is required, but it is also possible to use TES with conventional coaland nuclear-fired power plants which dominate the installed capacity of electricity utilities and are likely to continue to do so for the near future.