This review article discusses phase change memory (PCM), a non-volatile solid-state memory technology that uses the large electrical contrast between the highly-resistive amorphous and highly-conductive crystalline states of phase change materials. PCM has made rapid progress, surpassing older technologies in terms of scaling to small device dimensions and integrated large-array demonstrators with impressive retention, endurance, performance, and yield characteristics. The article surveys the current state of PCM, introduces the physics behind the technology, and assesses its characteristics in relation to various potential applications across the memory-storage hierarchy. It discusses the strengths of PCM, including scalability and rapid switching speed, and addresses challenges such as designing PCM cells for low RESET current, controlling device-to-device variability, and undesirable changes in the phase change material induced by fabrication. It also covers issues related to the operation of PCM devices, including retention, device-to-device thermal crosstalk, endurance, and bias-polarity effects. The article also addresses factors that can enhance PCM in the future, such as Multi-Level Cell (MLC) technology, the role of coding, and possible routes to ultra-high density PCM technology. The paper is organized into seven sections, beginning with the motivation for PCM, followed by an overview of PCM technology, the physics behind PCM, factors affecting the design and fabrication of PCM devices, issues related to the operation of PCM devices, factors that can enhance PCM in the future, and a conclusion. The article also discusses the potential applications of PCM in various memory technologies, including SRAM, DRAM, Flash, and Storage-Class Memory (SCM). It highlights the importance of PCM in the memory-storage hierarchy, its advantages over other non-volatile memory technologies, and its potential to replace existing technologies in the future. The article concludes with a discussion of the challenges and opportunities for PCM technology in the future.This review article discusses phase change memory (PCM), a non-volatile solid-state memory technology that uses the large electrical contrast between the highly-resistive amorphous and highly-conductive crystalline states of phase change materials. PCM has made rapid progress, surpassing older technologies in terms of scaling to small device dimensions and integrated large-array demonstrators with impressive retention, endurance, performance, and yield characteristics. The article surveys the current state of PCM, introduces the physics behind the technology, and assesses its characteristics in relation to various potential applications across the memory-storage hierarchy. It discusses the strengths of PCM, including scalability and rapid switching speed, and addresses challenges such as designing PCM cells for low RESET current, controlling device-to-device variability, and undesirable changes in the phase change material induced by fabrication. It also covers issues related to the operation of PCM devices, including retention, device-to-device thermal crosstalk, endurance, and bias-polarity effects. The article also addresses factors that can enhance PCM in the future, such as Multi-Level Cell (MLC) technology, the role of coding, and possible routes to ultra-high density PCM technology. The paper is organized into seven sections, beginning with the motivation for PCM, followed by an overview of PCM technology, the physics behind PCM, factors affecting the design and fabrication of PCM devices, issues related to the operation of PCM devices, factors that can enhance PCM in the future, and a conclusion. The article also discusses the potential applications of PCM in various memory technologies, including SRAM, DRAM, Flash, and Storage-Class Memory (SCM). It highlights the importance of PCM in the memory-storage hierarchy, its advantages over other non-volatile memory technologies, and its potential to replace existing technologies in the future. The article concludes with a discussion of the challenges and opportunities for PCM technology in the future.