Graphical User Interface: One of the motivations behind the adoption of tcl/tk as an embedded scripting shell was the ability to add a tk-based GUI. While our resources have not been sufficient to add the GUI at the initial release, we expect that the GUI will  be based on the Iwidgets set that is part of the [incr tcl] package and which are used in HEASARC's fv FITS viewer tool. Early code for XSPEC's GUI has already demonstrated that the switching capability described in the iostreams page performs as intended.

Later releases will be able to add support for new missions with differently formatted data and perhaps different response characterization that map detector channels to energy by other means than matrix convolution. Support for new fitting algorithms and statistics can be added with less risk to the overall development as they can be enabled with no change to the base code.

However, despite the large changes to the code, the program can continue to develop as it has done to date: by adding new theoretical models, fitting and statistical algorithms, and so on. We believe that XSPEC's useful life to the astronomical community has been significantly extended by the changes.

Examples of planned future extensions are:

• Completion of Bayesian Statistics package
• Markov Chain Monte Carlo methods to determine the posterior distribution of parameters
• Improved table models, extending the current standard
• Better tools and models for high resolution spectroscopy
• A mechanism for storing results into FITS files.