The SKYMAP Star Catalog System consists of a Master Catalog stellar database and a collection of utility software designed to create and maintain the database and to generate derivative mission star catalogs (run catalogs). It contains an extensive compilation of information on almost 300000 stars brighter than 8.0 mag. Introduction: The original SKYMAP Master Catalog was generated in the early 1970's. Incremental updates and corrections were made over the following years but the first complete revision of the source data occurred with Version 4.0. This revision also produced a unique, consolidated source of astrometric information which can be used by the astronomical community. The derived quantities were removed and wideband and photometric data in the R (red) and I (infrared) systems were added. This version, SKYMAP SKY2000 Master Catalog, Version 2 of the SKY2000 Master Catalog was completed in September of 1998 and represents a substantial improvement over Version 1 (see catalog <5095>). A high proportion of the astrometric data in Version 2 comes from ESA's Hipparcos mission, by way of the Hipparcos Output, Hipparcos Component, and Tycho Catalogues. Astrometric data are also present from USNO's ACT Catalog and the European Tycho Reference Catalogue, both of which refine the proper motions in the original Tycho catalog by an order of magnitude. While some objects still retain ACRS and PPM data, the proportion is very small and there are no stars left in the catalog which do not have astrometric quality positions. A large number of photoelectric data on the Johnson system (V, B) has been added from the Tycho Catalogue, such that there are now very few stars in SKY2000 that lack photoelectric data. More than 4000 stars observed by the Ball CT-601 star trackers on the RXTE spacecraft now have observed CCD ST magnitudes in Version 2. IAU designations based on J2000.0 positions have been added to the catalog, while all previous identifiers (SKY2000, HD, SAO, DM, HR, WDS, PPM, AG, bright- and variable-star designations) have been retained. The IAU-approved identifiers allow new objects to be inserted without disrupting the natural order of the principal catalog identifier. There are some deficiencies that exist in the SKY2000 Master Catalog that we are aware of. These items will be researched and corrected in future SKY2000 Master Catalog releases. (See file "deficent.txt").