This chapter describes the operation of various forms of phase‐change memristive devices and systems, evaluating their fundamental behaviour and outlines their possible advantages and disadvantages compared to more usual implementations. It introduces and demonstrates experimentally a new and simple approach to providing non‐volatile logic operations, specifically AND and OR Boolean logic, using phase‐change devices. Chalcogenide materials enables several novel device concepts, such as phase change memories (PCMs), optical disks and electronic threshold switches for high‐current select devices in crossbar arrays. The chapter proposes an integrated, all‐photonic memory employing phase‐change materials in combination with silicon nitride waveguides. It shows that it is possible to have sub‐nanosecond, high speed, multi‐level memories in nanophotonic circuits employing the commonly …