The intermittent nature of solar energy sources is the greatest challenge to the broad acceptance of this technology. The storage of thermal energy presents a workable option for addressing this issue. When it comes to the storage of thermal energy, latent heat storage units (LHSU) that make use of phase change materials (PCMs) are more desirable than sensible heat storage. In the context of a large increase in the demand for energy, PCMs are an essential class of thermal energy storage materials that contribute to the sustainable growth of both the economy and society. It stores large amounts of heat in the form of latent heat at a constant temperature. This promising technique has already been applied with great success in a variety of applications like solar appliances, buildings, battery thermal management, electronic cooling, waste heat recovery systems, textiles, and more. This book presents an in-depth discussion on PCMs, the current state of PCM technology, and a detailed description of their prospective applications.