This chapter includes an introduction to thermal energy storage systems. It lists the areas of application of the storage. It also includes the different storage systems; sensible, latent, and chemical. It concentrates on the concept and the application of latent thermal storage. A detailed overview of the energy storage capacity of latent systems is discussed. The motivation and the challenge to incorporate phase change materials in the storage system are highlighted. Next, a classification of different phase change materials (PCMs) and their applicability in different temperature ranges of operations are analyzed. A thorough review will be presented for its industrial applications.