It has recently become feasible to run personal digital assistants on phones and other personal devices. In this paper, we describe a design for a natural language understanding system that runs on-device. In comparison to a server-based assistant, this system is more private, more reliable, faster, more expressive, and more accurate. We describe what led to key choices about architecture and technologies. For example, some approaches in the dialog systems literature are difficult to maintain over time in a deployment setting. We hope that sharing learnings from our practical experiences may…Apple Machine Learning Research