Beginning with v10.00, XSPEC uses for its command line interface the command language Tcl. Because Tcl is a full scripting language, this interface allows users to write complex scripts with loops, branching, etc., which utilize XSPEC commands. This section covers those features of Tcl necessary to give the user similar functionality to that available in previous versions of XSPEC. It also includes some information on the details of our Tcl implementation that may be useful to experienced Tcl users. For a complete description of the Tcl command language, see for example ``Practical Programming in Tcl and Tk'', B. Welch, 1995, Prentice Hall.