This chapter first discusses the unique properties of phase change materials including the structure of the amorphous and crystalline phases as well as their optical, electrical and scaling properties. Then the principles of phase change memory (PCM) are described, including device characteristics, cell concepts and design, advanced prototype devices and scaling and process integration. We include a review on materials engineering for PCM and shows how materials can be optimized for various PCM applications. Finally, an outlook is given and new applications of phase change materials, such as such as logic and neuromorphic computing, are discussed.