Publisher Summary This chapter discusses the basic concepts involved in writing graphical user interfaces (GUIs), including figure files, application M-files, and callback functions. Modern users have grown accustomed to more sophisticated interaction with programs, through windows containing menus, buttons, drop-down lists, and the like. This way of interacting is by means of a GUI, as opposed to a text user interface (TUI) that uses command-line based input statements. MATLAB has a facility called GUIDE (graphical user interface development environment) for creating and implementing GUIs. GUIDE generates two files that save and launch the GUI: a FIG-file and an application M-file. The FIG-file contains a complete description of the GUI figure, that is, whatever controls and axes are needed for graphical interface. The application M-file contains the functions required to launch and control the GUI. The GUI is controlled largely by callback functions, which are subfunctions in the application M-file. These functions are written by the programmer who also specifies the components' actions.