MG11 
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High precision astrometry with the ESA mission Gaia.

The European Space Agency has formally approved in spring 2006 the overall funding of its next space astrometry mission Gaia scheduled for a launch in 2011. The mission will create an extraordinarily precise three-dimensional map of about one billion stars throughout our Galaxy and beyond from repeated astrometric and photometric observations over about five years. With an astrometric accuracy in the realm of the microarcsec the mission is the first instance of global astrometry in which a full an detailed relativistic modelling is mandatory to provide a consistent interpretation of the results. I will give a summary of the mission overall principles and show its relevance for accurate testing of the weak-field approximation of the gravitation theory thanks to the global mapping of the light deflection by solar system bodies and with the orbit determination of hundreds of PPN sensitive asteroids. Finally I will indicate also how the extensive zero-proper motion survey of thousands of quasars will allow to build a direct realization of the quasi-inertial celestial reference frame in the optics.