  The performance expected from a galaxy survey to be carried out with GAIA,
the GAIA Galaxy Survey, is outlined. From a statistical model of galaxy number
density, size and surface brightness distribution, and from detailed numerical
simulations based on real images, it is conservatively estimated that GAIA
would be able to detect and observe about 3 million galaxies brighter than $V
\simeq 17$ and to provide multi-colour and multi-epoch broad-band photometry of
these with an end-of-mission angular resolution of $\simeq$ 0.35 \as and a
photometric accuracy of $simeq$ 0.2 {mag/arcsec$^2$} at $\mu_V = 20$
{mag/arcsec$^2$}. The substantial scientific case for performing such a survey
and the additional efforts required in terms of mission preparation, operations
and telemetry are also discussed.
