This paper provides an overview of the science objectives of the Gaia mission, with deeper snapshots on specific areas not particularly covered in this volume. Starting with the expected performances 1 show how the combined and repeated information on each source will lead to a giant leap in our current knowledge of stellar and galactic physics, resulting both from the accuracy on individual objects but also from the statistical exploitation of the unbiased census over one billion stars. I then draw attention to the additional science output of the mission like the detection of extra-solar planetary systems, the determination of thousands of stellar masses, the massive impact on Solar System studies, the testing of the Einstein relativity theory to an unprecedented level together with the direct realisation of the primary inertial frame in the visible.
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