Object classification and the determination of stellar parameters with   Gaia
  Gaia will observe more than one billion objects brighter than V=20, including stars, asteroids, galaxies and quasars. As Gaia performs real time detection (i.e. without an input catalogue) the intrinsic properties of most of these objects will not be known a priori. An integral part of the Gaia data processing is therefore to classify everything observed. This will be based primarily on multiband photometry provided by Gaia, but should also make optimal use of the high resolution spectroscopy (for brighter stars) and the parallaxes. In addition to a broad classification, we can also determine fundamental stellar parameters, in particular effective temperature, metallicity and the line-of-sight interstellar extinction. Such information will be essential for fully exploiting the astrometric part of the Gaia catalogue for stellar population studies. However, extracting this information is a significant challenge, and will need to make use of appropriate multidimensional data analysis techniques. I outline some of the problems and the strategies being developed to tackle them. 
C.A.L. Bailer-Jones