Phase-change materials exhibit some remarkable properties. They can be crystallized in picoseconds and amorphized in femtoseconds, but remain stable against spontaneous crystallization for many years. They exhibit huge differences in optical and electrical properties between the amorphous and crystalline phases. Such properties have led, over the last four decades, to the successful development of both optical and electrical non-volatile phase-change memories. However, such binary memories only ‘scratch the surface’ in terms the remarkable potential applications of phase-change materials and devices, which extend to arithmetic, logical and bio-inspired (or neuromorphic) processing. In this paper we introduce and explain some of this remarkable functionality inherent to phase-change systems. Phase-change device operating in accumulation mode. This provides the basic mechanism for providing arithmetic, logic and neuronal type processing.