GemTcl (Gtk Embedded Tcl) is a cross platform Tcl shell written in Gtk. In addition to interactive use, it supports the creation of buttons that may contain user-defined Tcl scripts. GemTcl also supports the loading of Glade interfaces for which the callbacks of their buttons may be edited. It may thus be considered as a rapid application development environment. It is especially suited for control applications, and comes with an XML-RPC controlled microscope simulator application to exemplify that.