The Synergy between VLBI and Gaia astrometry
  With the publication of Gaia DR2, 1.3 billion stars now have public parallax
and proper motion measurements. In this contribution, we compare the results
for sources that have both optical and radio measurements, focusing on
circumstellar masers. For these large, variable and bright AGB stars, the VLBI
astrometry results can be more robust. Moreover, there are a number of
applications where VLBI astrometry provides unique data for studying stellar
populations and Galactic structure. The BeSSel project not only provides
parallax and proper motions at much larger distances than Gaia can reach, but
it also uniquely samples the spiral arms of the Galaxy. The evolved stars in
the BAaDE sample can potentially constrain the dynamics and stellar content of
the inner bulge and bar of the Milky Way, not reachable in the optical.
