Phase Change Materials are solids which are characterized by a unique combination of properties. They exist in an amorphous and a crystalline phase with remarkably different optical and electrical properties caused by an unusual change of bonding when the amorphous phase is crystallized. It is possible to change the phase of such a material in very short times (nanoseconds) and repeatedly between the two phases which makes phase change materials ideal candidates for data storage. This paper reviews in detail the relationship between the bonding mechanisms and the resulting physical properties of phase change materials. It describes the change of bonding from ordinary covalent bonding in the amorphous phase to resonance bonding in the crystalline phase with additional disorder, resulting in unconventional physical properties of phase change materials. These properties lead to the development of phase change data storage applications. Phase change optical data storage, phase change random access memory, and emerging applications including neuromorphic computing are described with particular emphasis on material requirements and material engineering for phase change random access memory.