  The body of photometric and astrometric data on stars in the Galaxy has been
growing very fast in recent years (Hipparcos/Tycho, OGLE-3, 2-Mass, DENIS,
UCAC2, SDSS, RAVE, Pan Starrs, Hermes, ...) and in two years ESA will launch
the Gaia satellite, which will measure astrometric data of unprecedented
precision for a billion stars. On account of our position within the Galaxy and
the complex observational biases that are built into most catalogues, dynamical
models of the Galaxy are a prerequisite full exploitation of these catalogues.
On account of the enormous detail in which we can observe the Galaxy, models of
great sophistication are required. Moreover, in addition to models we require
algorithms for observing them with the same errors and biases as occur in real
observational programs, and statistical algorithms for determining the extent
to which a model is compatible with a given body of data.
  JD5 reviewed the status of our knowledge of the Galaxy, the different ways in
which we could model the Galaxy, and what will be required to extract our
science goals from the data that will be on hand when the Gaia Catalogue
becomes available.
