  Gaia will provide parallaxes and proper motions with accuracy ranging from 10
to 1000 microarcsecond on up to one billion stars. Most of these will be disk
stars: for an unreddened K giant at 6 kpc, it will measure the distance
accurate to 15% and the transverse velocity to an accuracy of about 1 km/s.
Gaia will observe tracers of Galactic structure across the whole HR diagram,
including Cepheids, RR Lyrae, white dwarfs, F dwarfs and HB stars. Onboard low
resolution spectrophotometry will permit -- in addition to a Teff estimate --
dwarf/giant discrimination, metallicity measurement and extinction
determination. For the first time, then, Gaia will provide us with a 3D
spatial/properties map and at least a 2D velocity map of these tracers (RVs
will be obtained too for brighter stars.) This will be a goldmine of
information from which to learn about the origin and evolution of the Galactic
disk. I briefly review the Gaia mission, and then show how the expected
astrometric accuracies translate into distance and velocity accuracies and
statistics. I examine the impact Gaia should have on a few scientific areas
relevant to the Galactic disk. I discuss how a better determination of the
spiral arm locations and pattern speed, plus a better reconstruction of the
Sun's orbit over the past billion years (from integration through the
Gaia-measured gravitational potential) will allow us to assess the possible
role of spiral arm crossings in ice ages and mass extinctions on the Earth.
