  Gaia is a fully-approved all-sky astrometric and photometric survey due for
launch in 2011. It will measure accurate parallaxes and proper motions for
everything brighter than G=20 (ca. 10^9 stars). Its primary objective is to
study the composition, origin and evolution of our Galaxy from the 3D
structure, 3D velocities, abundances and ages of its stars. In some respects it
can be considered as a cosmological survey at redshift zero. Several other
upcoming space-based surveys, in particular JWST and Herschel, will study star
and galaxy formation in the early (high-redshift) universe. In this paper I
briefly describe these missions, as well as SIM and Jasmine, and explain why
they need to observe from space. I then discuss some Galactic science
contributions of Gaia concerning dark matter, the search for substructure,
stellar populations and the mass--luminosity relation. The Gaia data are
complex and require the development of novel analysis methods; here I summarize
the principle of the astrometric processing. In the last two sections I outline
how the Gaia data can be exploited in connection with other observational and
theoretical work in order to build up a more comprehensive picture of galactic
evolution.
