Maintenance of the link to Hipparcos

The optical wavelength astrometric reference frame is currently based on observations made by the Hipparcos astrometric satellite. Unfortunately, since the Hipparcos Celestial Reference Frame (HCRF) is comprised of bright stars which are sparsely distributed on the sky, the frame often is inappropriate for astronomical and astrometric programs requiring a denser distribution of faint stars. Thus, significant eort has been channeled into the densification of the HCRF. We discuss optical reference frame densification programs and the maintenance of the link between these programs and the HCRF.