A Phase Change Material (PCM) is a substance with a high heat of fusion which, melting and solidifying at certain temperatures, is capable of storing or releasing large amounts of energy.

The only phase change used for PCMs is the solid-liquid change. Liquid-gas PCMs are not practical for use as thermal storage due to the large volumes or high pressures required to store the materials when in their gas phase Initially, the solid-liquid PCMs perform like conventional storage materials; their temperature rises as they absorb heat.