  The current state of the link problem between radio and optical celestial
reference frames is considered. The main objectives of the investigations in
this direction during the next few years are the preparation of a comparison
and the mutual orientation and rotation between the optical {\it Gaia}
Celestial Reference Frame (GCRF) and the 3rd generation radio International
Celestial Reference Frame (ICRF3), obtained from VLBI observations. Both
systems, ideally, should be a realization of the ICRS (International Celestial
Reference System) at micro-arcsecond level accuracy. Therefore, the link
accuracy between the ICRF and GCRF should be obtained with similar error level,
which is not a trivial task due to relatively large systematic and random
errors in source positions at different frequency bands. In this paper, a brief
overview of recent work on the GCRF--ICRF link is presented. Additional
possibilities to improve the GCRF--ICRF link accuracy are discussed. The
suggestion is made to use astrometric radio sources with optical magnitude to
20$^m$ rather than to 18$^m$ as currently planned for the GCRF--ICRF link. In
addition, the use of radio stars is also a prospective method to obtain
independent and accurate orientation between the Gaia frame and the ICRF.
