Widget Toolkit System - Widget Library - GUI Toolkit - UX Library is a library or a collection of libraries containing a set of graphical control elements (called widgets) used to construct the graphical user interface (GUI) of programs Types TK (Tickle Tackle) Tcl is a general purpose multi-paradigm system programming language Tk is a cross platform widget toolkit used for building GUI in many languages Flutter a Google created UI SDK Qt Toolkit is cross-platform application development framework for creating graphical user interfaces GTK Toolkit is a free and open-source cross-platform widget toolkit for creating graphical user interfaces