This library implements a full callback system for use in widget libraries, abstract interfaces, and general programming. Originally part of the Gtk-- widget set, libsigc++20 is now a separate library to provide for more general use. It is the most complete library of its kind with the ability to connect an abstract callback to a class method, function, or function object. It contains adaptor classes for connection of dissimilar callbacks and has an ease of use unmatched by other C++ callback libraries. Package GTK-- (gtkmm), which is a C++ binding to the GTK+ library, starting with version 1.1.2, uses libsigc++20.