MathUI is an international workshop to discuss how users can be best sup- ported when doing, learning communicating with, and searching for mathematics digitally, in short, interacting with mathematical content using digital means. Our leading questions are the following: • What do we know about interactions between users and math? • Which mathematical services can be ofered, and can they be meaningfully combined? • How is mathematics for which purpose best represented? • What specifically math-oriented support or platforms are needed? • How can we exploit best practices concerning mathematics for better math-user interactions? On the 31st July, 2021, the 13th instance of MathUI was gracefully hosted online by the West University of Timisoara, Romania. A warm thank you to our organizers, chairs, reviewers, and last but not least authors! Andrea Kohlhase