Abstract: sml_tk: Functional Programming for GUIs - Reference Manual Christoph Lüth and Burkhart Wolff In this reference manual, we describe the SML-based programming environment sml_tk for graphical user interfaces, version 3.0. sml_tk is based on the highly portable X-Window Toolkit Tk (and uses internally the Tcl/Tk interpreter wish), but offers functional abstraction over Tk and an own component library for graphical standard widgets such as info-boxes, treelist-widgets, tabs and tables. sml_tk is the basic library for a collection of GUIs for formal method tools such as TAS and IsaWin.