  The Milky Way is made up of a central bar, a disk with embedded spiral arms,
and a dark matter halo. Observational and theoretical constraints for the
characteristic parameters of these components will be presented, with emphasis
on the constraints from the dynamics of the Milky Way gas. In particular, the
fraction of dark matter inside the solar radius, the location of the main
resonances, and the evidence for multiple pattern speeds will be discussed.
