The Guide Star Catalog II (GSC-II) is a joint project of the STScI (Baltimore) and INAF-OATo (Italy) for the construction of an all-sky astronomical catalogue containing position, multicolour photometry, classification and proper motions for 1 billion of stars and galaxies down to R˜20 mag. Astrometric and photometric data have been derived from the analysis and calibrations of about 8000 digitized Schmidt plates of the Palomar and AAO surveys. The last release, GSC 2.3, contains positions, photographic photometry B,R,I, and classification for approximately 997,028,547 entries extracted from the complete GSC-II database. GSC 2.3 is currently used for HST Bright Object Protection and it replaces GSC-I for HST pointing since cycle 15. Public access to this catalog is available through a web form at STScI and OATo, or via the services provided by the National VO and the Italian VO. Here we describe the properties of GSC 2.3 and discuss its astrometric and photometric accuracy with respect the other large surveys.