The article provides a comprehensive overview of phase change memory (PCM) technology, a non-volatile solid-state memory that leverages the significant electrical contrast between the highly resistive amorphous and highly conductive crystalline states in phase change materials. PCM has shown 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 authors introduce the physics behind PCM, including the scalability of phase change materials and the physical processes affecting switching speed. They discuss the design and fabrication of PCM devices, addressing challenges such as low RESET current, device-to-device variability, and fabrication-induced changes in the phase change material. The article also covers operational issues like retention, device-to-device thermal crosstalk, endurance, and bias-polarity effects. Future enhancements to PCM are explored, including Multi-Level Cell (MLC) technology, coding, and routes to ultra-high density. The authors conclude by discussing the potential applications of PCM across various memory-storage hierarchies, such as SRAM, DRAM, Flash, and Storage-Class Memory (SCM), and highlight the need for PCM to overcome scaling challenges and improve endurance and write performance to compete with established technologies. Overall, the article emphasizes the potential of PCM to address the limitations of current memory technologies and its role in advancing the memory/storage hierarchy in computing platforms.The article provides a comprehensive overview of phase change memory (PCM) technology, a non-volatile solid-state memory that leverages the significant electrical contrast between the highly resistive amorphous and highly conductive crystalline states in phase change materials. PCM has shown 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 authors introduce the physics behind PCM, including the scalability of phase change materials and the physical processes affecting switching speed. They discuss the design and fabrication of PCM devices, addressing challenges such as low RESET current, device-to-device variability, and fabrication-induced changes in the phase change material. The article also covers operational issues like retention, device-to-device thermal crosstalk, endurance, and bias-polarity effects. Future enhancements to PCM are explored, including Multi-Level Cell (MLC) technology, coding, and routes to ultra-high density. The authors conclude by discussing the potential applications of PCM across various memory-storage hierarchies, such as SRAM, DRAM, Flash, and Storage-Class Memory (SCM), and highlight the need for PCM to overcome scaling challenges and improve endurance and write performance to compete with established technologies. Overall, the article emphasizes the potential of PCM to address the limitations of current memory technologies and its role in advancing the memory/storage hierarchy in computing platforms.