The Magic User Interface (MUI in short) is an object-oriented system by Stefan Stuntz to generate and maintain graphical user interfaces. With the aid of a preferences program, the user of an application has the ability to customize the system according to personal taste. The Magic User Interface was written for AmigaOS and gained popularity amongst both programmers and users. It has been ported to PowerPC processors and adopted as the default GUI toolkit of the MorphOS operating system. The MUI application programmer interface has been cloned by the Zune toolkit used in the AROS Research Operating System. History Creating GUI applications on Amiga was difficult for a very long time, mainly because the programmer got only a minuscule amount of support from the operating system. Beginning with Kickstart 2.0, the gadtools.library was a step in the right direction, however, even using this library to generate complex and flexible interfaces remained difficult and still required a great deal of patience. The largest problem in existing tools for the creation of user interfaces was the inflexible output. Most of the programs were still using built-in fonts and window sizes, thus making the use of new high resolution graphics hardware adapters nearly unbearable. Even the preference programs on the Workbench were still only using the default fixed-width font. In 1992 Stefan Stuntz started developing a new object-oriented GUI toolkit for Amiga. Main goals for new GUI toolkit were: Font sensitivity: Possible for the font to be set in every application. Changeable window sizes: Windows have a sizing gadget which allows users to change the window size until it suits their needs Flexibility: Elements can be changed by the user regarding their own personal tastes. Controlling by keyboard: Widgets can be controlled by the keyboard as well as by the mouse. System integration: Every program has an ARexx port and can be iconified or uniconified by pushing a gadget or by using the Commodities exchange program. Adjusting to its environment: Every application can be made to open on any screen and adapts itself to its environment. MUI was released as shareware. Starting from MUI 3.9 an unrestricted version is integrated with MorphOS, but a shareware key is still required to activate all user configuration options in AmigaOS. Application theory UI development is done at source-code level without the aid of GUI builders. In MUI application the programmer only defines logical structure of the