  The only way to map the Galaxy's gravitational potential $\Phi({\bf x})$ and
the distribution of matter that produces it is by modelling the dynamics of
stars and gas. Observations of the kinematics of gas provide key information
about gradients of $\Phi$ within the plane, but little information about the
structure of $\Phi$ out of the plane. Traditional Galaxy models {\em assume},
for each of the Galaxy's components, arbitrary flattenings, which together with
the components' relative masses yield the model's equipotentials. However, the
Galaxy's isopotential surfaces should be {\em determined\/} directly from the
motions of stars that move far from the plane. Moreover, from the kinematics of
samples of such stars that have well defined selection criteria, one should be
able not only to map $\Phi$ at all positions, but to determine the distribution
function $f_i({\bf x},{\bf v})$ of each stellar population $i$ studied. These
distribution functions will contain a wealth of information relevant to the
formation and evolution of the Galaxy. An approach to fitting a wide class of
dynamical models to the very heterogeneous body of available data is described
and illustrated.
