In the last two decades, thermal energy storage (TES) has gained wide popularity as it can effectively bridge the demand-supply gap. Phase change materials (PCMs) are the most effective of the TES methods due to their high energy density under nearly isothermal conditions. The simple phase change observed is the solid to liquid phase, such as paraffin wax, which absorbs thermal energy and continues melting at a constant temperature until it completely melts. The organic esters, polyethylene glycols, inorganic salts, eutectic mixture, etc. are the prominent PCMs. Organic PCMs and, particularly, poly(ethylene glycol) (PEG) can be extremely resourceful when preparing a wide array of polymers for thermal energy storage. By preparing different polymers, a plethora of working temperatures can be achieved to work for the user’s comfort. After considering the solid-liquid transition, the techniques required to contain the PCMs are discussed. Various material combinations based on chemical and physical methods are presented, which are adapted to form solid-solid PCMs. Both solid-solid phase change and thermal conductivity are discussed as the applications depend on the fast exchange of heat. Normally, inorganic blend components provide higher thermal conductivity compared to their organic counterparts. The chapter discusses the chemical reaction of a functional group with a PCM chemical structure, which can be achieved with a large number of reactive molecules to form polymeric PCMs, such as polyester, epoxy, polyurethane, etc. Finally, specific application possibilities are explored.