The authors survey the current state of phase change memory (PCM), a nonvolatile solid-state memory technology built around the large electrical contrast between the highly resistive amorphous and highly conductive crystalline states in so-called phase change materials. PCM technology has made rapid progress in a short time, having passed older technologies in terms of both sophisticated demonstrations of scaling to small device dimensions, as well as integrated large-array demonstrators with impressive retention, endurance, performance, and yield characteristics. They introduce the physics behind PCM technology, assess how its characteristics match up with various potential applications across the memory-storage hierarchy, and discuss its strengths including scalability and rapid switching speed. Challenges for the technology are addressed, including the design of PCM cells for low reset current, the need to control device-to-device variability, and undesirable changes in the phase change material that can be induced by the fabrication procedure. They then turn to issues related to operation of PCM devices, including retention, device-to-device thermal cross-talk, endurance, and bias-polarity effects. Several factors that can be expected to enhance PCM in the future are addressed, including multilevel cell technology for PCM (which offers higher density through the use of intermediate resistance states), the role of coding, and possible routes to an ultrahigh-density PCM technology.