MUI (Magic User Interface) is an object-oriented graphical user interface toolkit for AmigaOS that superseded the older BOOPSI system. Developed by Stefan Stuntz, it provides modern, customizable widgets with theme support through external metafile-based configuration, allowing interface changes without recompilation. It became the standard GUI framework for AmigaOS 3.x and MorphOS, while Zune serves as its open-source reimplementation for the AROS operating system.