In this compilation, after considering solid-liquid transition, techniques required to obtain phase change materials are discussed. Various material combinations based on chemical and physical methods are also discussed, which are adopted to form solid-solid phase change. Following this, a non-parity-time-symmetric three-layer structure is introduced, consisting of a gain medium layer sandwiched between two phase-change medium layers for switching the direction of reflectionless light propagation. The concluding chapter discusses the effectiveness of phase change materials in building roofs...