  Our Galaxy is a complex machine in which several processes operate
simultaneously: metal-poor gas is accreted, is chemically enriched by dying
stars, and then drifts inwards, surrendering its angular momentum to stars; new
stars are formed on nearly circular orbits in the equatorial plane and then
diffuse through orbit space to eccentric and inclined orbits; the central
stellar bar surrenders angular momentum to the surrounding disc and dark halo
while acquiring angular momentum from inspiralling gas; the outer parts of the
disc are constantly disturbed by satellite objects, both luminous and dark, as
they sweep through pericentre. We review the conceptual tools required to bring
these complex happenings into focus. Our first concern must be the construction
of equilibrium models of the Galaxy, for upon these hang our hopes of
determining the Galaxy's mean gravitational field, which is required for every
subsequent step. Ideally our equilibrium model should be formulated so that the
secular evolution of the system can be modelled with perturbation theory. Such
theory can be used to understand how stars diffuse through orbit space from
either the thin gas disc in which we presume disc stars formed, or the debris
of an accreted object, the presumed origin of many halo stars. Coupling this
understanding to the still very uncertain predictions of the theory of stellar
evolution and nucleosynthesis, we can finally extract a complete model of the
chemodynamic evolution of our reasonably generic Galaxy. We discuss the
relation of such a model to cosmological simulations of galaxy formation, which
provide general guidance but cannot be relied on for quantitative detail.
