GTK is a multi-platform toolkit for creating graphical user interfaces. Offering a complete set of widgets, GTK is suitable for projects ranging from small one-off projects to complete application suites.

GTK is free software and part of the GNU Project. However, the licensing terms for GTK , the GNU LGPL, allow it to be used by all developers, including those developing proprietary software, without any license fees or royalties.

GTK is based on three libraries developed by the GTK team:

- GLib is the low-level core library that forms the basis of GTK and GNOME. It provides data structure handling for C, portability wrappers, and interfaces for such runtime functionality as an event loop, threads, dynamic loading, and an object system.
- Pango is a library for layout and rendering of text, with an emphasis on internationalization. It forms the core of text and font handling for GTK -2.0.
- The ATK library provides a set of interfaces for accessibility. By supporting the ATK interfaces, an application or toolkit can be used with such tools as screen readers, magnifiers, and alternative input devices.

GTK has been designed from the ground up to support a range of languages, not only C/C . Using GTK from languages such as Perl and Python (especially in combination with the Glade GUI builder) provides an effective method of rapid application development.