STk is a free R4RS Scheme interpreter which can access the Tk graphical package. Concretely, it can be seen as the standard Tk package where Tcl has been replaced by a Scheme interpreter. STk embeds also an efficient CLOS like object oriented system, called STklos, which provides: multiple inheritance, generic functions, multi-methods, a MOP (Meta Object Protocol) STklos MOP is based on the Tiny C