Phase change memory devices were originally reported by S. R.Ovshinsky  in 1968. A 256-bit phase-change memory array based on chalcogenide materials was reported in 1970  Recent advances in phase change materials, memory device designs, and process technology have resulted in significant advances in phase change device performance, and a new memory device, called Ovonic Unified Memory (OUM), has been developed. This paper will discuss various device and materials characteristics of OUM phase change memory materials of interest in applications for nonvolatile high-density memories. These materials are generally Te chalcogenide based, exploiting the congruent crystallization of the FCC phase and the associated reduction in resistivity that results from crystallization from the quenched amorphous state. Data storage is a thermally initiated, rapid, reversible structural phase change in the film. While rewriteable DVD disks employ laser heat to induce the phase change and modulate reflectivity, OUM technology uses a short electrical current pulse to modulate resistivity. The device geometry and thermal environment dictate the power and energy required for memory state programming.