Abstract: A review is presented of phase-change semiconductor memories, including their principle of operation, major advantages, device structure design, phase-change materials, current research and key fabrication techniques. Due to its advantages of nonvolatility, high cycling capability, small cell size, low cell energy consumption, multilevel storage, high read rate, superior radiation/vibration/electron-disturbance tolerance, superior high/low temperature tolerance and simple cell structure with high scalability, C-RAMs have the highest potential to replace all kinds of current memory devices such as flash memories, dynamic random access memories, and static random access memories in the future.