  The GAIA Galactic survey satellite will obtain photometry in 15 filters of
over 10^9 stars in our Galaxy across a very wide range of stellar types. No
other planned survey will provide so much photometric information on so many
stars. I examine the problem of how to determine fundamental physical
parameters (Teff, log g, [Fe/H] etc.) from these data. Given the size,
multidimensionality and diversity of this dataset, this is a challenging task
beyond any encountered so far in large-scale stellar parametrization. I
describe the problems faced (initial object identification, interstellar
extinction, multiplicity, missing data etc.) and present a framework in which
they can can be addressed. A probabilistic approach is advocated on the grounds
that it can take advantage of additional information (e.g. priors and data
uncertainties) in a consistent and useful manner, as well as give meaningful
results in the presence of poor or degenerate data. Furthermore, I suggest an
approach to parametrization which can use the other information GAIA will
acquire, in particular the parallax, which has not previously been available
for large-scale multidimensional parametrization. Several of the problems
identified and ideas suggested will be relevant to other large surveys, such as
SDSS, DIVA, FAME, VISTA and LSST, as well as stellar parametrization in a
virtual observatory.
