n this paper, we come back on the history of celestial reference systems, until the Gaia era, and we lay the foundations of this research area. Gaia should revolutionize this domain by creating the first extragalactic celestial reference frame directly at optical bands, on the basis of more than 10000 QSOs (Quasi Stellar Objects). In parallel, the radio frame obtained with the VLBI (Very Long Baseline Interferometry) technique has been the IAU (International Astronomical Union) fundamental celestial reference frame for several decades. The alignment between these two frames will be important in the future, not only for guaranteeing a proper transition if the fundamental reference frame is moved from the radio to the optical domain, but also for registering the radio and optical images of any celestial target with the highest accuracy. This should be of high astrophysical interest, for example to probe properly the physics of active galactic nuclei (AGN) hence placing constraints on the overall AGN geometry. We will present the studies investigated so far (in the radio but also optical domains) in order to prepare the celestial reference frame for the Gaia era.< Leer menos