Gaia is an ESA mission to build a census of our Milky Way Galaxy. This is done with a suite of three instruments combining two fields of view into a single focal plane. On-board detection of all sources will ensure a bias free survey down to the 20th magnitude. In addition to astrometry also photometry is done down to the 20th magnitude. The third, spectroscopic instrument will provide radial velocities down to the 17th magnitude of the Galactic stellar population. Currently the scientific requirements on Gaia are largely met except for bright stars for which in astrometry additional efforts are needed to squeeze the remaining μarcsecs out of the positional information error budgets. In the mid-magnitude range (15 mag) the end of mission parallax errors are from 10 to 25 μarcsec and photometry errors are between 5 and 16 mmag. Radial velocities are better than 1 km s-1 for the bright stars with errors increasing to 15 km s-1 for the faintest ones. At the moment there are no unsolvable technical challenges on the table and Gaia is progressing toward the launch in 2012.