The Gaia space astrometric mission will construct its own celestial reference frame based on thousands of QSO positions measured with an accuracy of a few tens of microarcseconds. For consistency with the current realization of the International Celestial Reference Frame, which is based on Very Long Baseline Interferometry (VLBI) positions for more than 3000 extragalactic radio sources, it is crucial that the two frames be aligned with the highest possible accuracy. This alignment requires a number of common objects with excellent optical and radio astrometric properties. Because of their physical properties, it is possible that the QSO optical and radio positions do not physically coincide at the 100 microarcsecond level. While such shifts may affect the accuracy in aligning the two frames, they may also help gaining insights into the physical conditions in the nuclear regions of these objects. This paper reviews the ongoing work to align the VLBI and Gaia frames along with the potential astrophysical outcome that may result from comparing/combining ultra-accurately these two frames.