... For 30 years the number of X-ray sources has steadily grown as technology has improved, but there is still no definitive reference catalog of X-ray objects. Before the next wave of X-ray source catalogs appears, the time seems ripe to construct a unified X-ray catalog (XCAT) using the existing catalogs. The lack of a unified catalog is severely limiting our ability to identify X-ray sources, combine data across different missions, and conduct global multiwavelength studies. Catalogs produced by older missions are not superceded by the catalogs from newer missions. The instrument sensitivities and the energy range are different from mission to mission and contribute to a changing view of the X-ray sky. This unified list, XCAT, will be organized by object and contain links to all the information currently contained in the exisiting catalogs. Since the ROSAT sources domonate the known X-ray source population, our first step is to produce a merged master list of ROSAT sources. To fulfill this task we will 1) complete WGACAT for missing sequences and to correct minor problems, 2) merge the various ROSAT catalogs into a single object list, and 3) include the source lists produced by other past and present missions (e.g., ASCA, Einstein etc.). Optical and other identification parameters where available will be included. Our goal is to make a single list of objects, that includes links back to the original catalog information so that a complete history of each source can be obtained. The XCAT infrastructure will be designed to easily incorporate the larger number of objects expected from future X-ray observatories. ...