Gaia, a cornerstone-class mission, is the next global space astrometry initiative of the European Space Agency targeted to reach the 25 micro-arc-second level accuracy at mag 15. Its main goal is to make the largest, most precise three-dimensional map of our Galaxy by surveying for five years more than a billion stars down to V=20. The survey is complemented with simultaneous multi-epoch photometry and spectroscopy, including radial velocity measurements. Gaia's launch is just around the corner! Lift off is now scheduled for Nov 20th at 08:57 UTC from the French Guyana. The release of the final, full accuracy, science qualified archive to the community worldwide is then anticipated for early 2021. However, ESA has decided that early/intermediate quality data of limited accuracy but potentially suitable to address prime science will be released to the scientific community at large even during Gaia?s operational life. The talk will review the status of the mission and its organization, and recall the complexities of the global astrometric problem from space. It will then discuss the expected performances, in the light of the 'as-built' properties of the payload, and their impact on the mission science case with particular emphasis on the Milky Way as a testing ground of present-day predictions of cosmological models.