AXAF, the Advanced X-ray Astrophysics Facility will be the third satellite in the series of great observatories in the NASA program, after the Hubble Space Telescope and the Gamma Ray Observatory. At launch in fall 1998, AXAF will carry a high resolution mirror, two imaging detectors, and two sets of transmission gratings (Holt 1993). Important AXAF features are: an order of magnitude improvement in spatial resolution, good sensitivity from 0.1-10keV, and the capability for high spectral resolution observations over most of this range. The Tycho Catalogue from the Hipparcos mission will serve as a primary part of the AXAF Guide and Aspect Star Catalog (AGASC), particularly as merged with other star catalogues. Incorporation of the Tycho Catalogue into AGASC will provide: (1) accurate positions and magnitudes for target acquisition and guiding; (2) the colours necessary for magnitude transformations to the AXAF Aspect Camera (ACA) system; (3) high internal astrometric accuracy for half-arcsecond post-facto image reconstruction; and (4) an external astrometric system that will tie AXAF X-ray image astrometry into the best positional system expected well beyond the mission lifetime and into the next millennium.