XPA Messaging provides seamless communication between Unix programs, including X, Tcl/Tk, and generic select() loops. It provides an easy way for users to communicate with these XPA-enabled programs by executing XPA client commands from a shell. Because XPA works both at the programming level and the shell level, it is a powerful tool for unifying any environment, offering flexibility in choosing the best level at which to access XPA services. A program becomes an XPA-enabled server by defining named points of public access through which data and commands can be exchanged with other client programs (and users). Using standard TCP sockets, XPA supports both single-point and broadcast messaging to and from these servers.