Phase change materials (PCMs) can store and release heat, while they are changing phase. They allow relatively large amount of energy to be stored in comparatively small volumes. Due to their attractive and controllable properties, they are useful in many applications. For instance, PCMs have been used as smart building materials, energy storage devices, heat recovering agents, etc. By virtue of their properties, these materials have also found applications in smart textiles and aerospace industry. This book aims to review the synthesis and properties of a variety of functional organic and inorganic phase change material systems developed in the recent years for advanced applications.