Astrometry from space has unique advantages over ground-based observations: the all-sky coverage, relatively stable and temperature- and gravity-invariant operating environment delivers precision, accuracy and sample volume several orders of magnitude greater than ground-based results. Even more importantly, absolute astrometry is possible. The European Space Agency Cornerstone mission Gaia is delivering that promise. Gaia provides 5-D phase space measurements, 3 spatial coordinates and two space motions in the plane of the sky, for a representative sample of the Milky Way’s stellar populations (over 1 billion stars, being ~1% of the stars over 50% of the volume). Full 6-D phase space data is delivered from line-of-sight (radial) velocities for the 300million brightest stars. These data make substantial contributions to astrophysics and fundamental physics on scales from the Solar System to cosmology. Deriving full value requires reliable supplementary information, especially on stellar chemical abundances. The Gaia-ESO Public Spectroscopic Survey is an example of such complementary projects, with VLT spectra for 100,000 stars. Gaia-ESO uses very many abundance analysis methods to determine both random and systematic uncertainties in stellar abundances. An overview of Gaia-ESO and some of the challenges in what one can believe will be given.