Phase change materials (PCMs) can exist in at least two different phases (an amorphous and one or more crystalline phases), and they can be switched repeatedly between these phases. The different phases have distinctly different physical properties such as electrical conductivity, optical reflectivity, mass density, or thermal conductivity. These differences and the repeatability of the switching give these materials the ability to store information. Rewritable compact discs, digital versatile discs, and Blu-ray™ discs store information in thin films of PCMs, using the difference in reflectivity between the phases as the storage mechanism. A novel solid-state memory technology that applies the difference in electrical resistivity is currently being developed, and possible future applications include reconfigurable logic. This article reviews the unique set of properties of PCMs and their current and future applications.