  The GAIA mission will produce an extraordinary database from which we should
be able to deduce not only the Galaxy's current structure, but also much of its
history, and thus cast a powerful light on the way in which galaxies in general
are made up of components, and of how these formed. The database can be fully
exploited only by fitting to it a sophisticated model of the entire Galaxy.
Steady-state models are of fundamental importance even though the Galaxy cannot
be in a steady state. A very elaborate model of the Galaxy will be required to
reproduce the great wealth of detail that GAIA will reveal. A systematic
approach to model-building will be required if such a model is to be
successfully constructed, however. The natural strategy is to proceed through a
series of models of ever increasing elaborateness, and to be guided in the
specification of the next model by mismatches between the data and the current
model.
  An approach to the dynamics of systems with steady gravitational potentials
that we call the `torus programme' promises to provide an appropriate framework
within which to carry out the proposed modelling programme. The basic
principles of this approach have been worked out in some detail and are
summarized here. Some extensions will be required before the GAIA database can
be successfully confronted. Other modelling techniques that might be employed
are briefly examined.
