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 for the reduction of energy consumption and the improvement of indoor comfort conditions.