This final chapter deals with the use of latent heat to store thermal energy. Latent heat is associated with phase transitions and the materials that are used to store energy related to latent heat are referred to as phase change materials. Two common phase transitions that are used for latent heat energy storage are the solid to liquid transition, such as the melting transition of water, and the liquid to gas transition, such as the boiling transition of liquid air. Applications of these methods include the heating of office buildings and the storage of excess electricity for grid use. The chapter also considers thermochemical energy storage where energy is stored using reversible chemical reactions or by adsorption or absorption processes.