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Gaia will provide parallaxes and proper motions with accuracy ranging from 10to 1000 microarcsecond on up to one billion stars. Most of these will be diskstars: for an unreddened K giant at 6 kpc, it will measure the distanceaccurate 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 lowresolution spectrophotometry will permit -- in addition to a Teff estimate --dwarf/giant discrimination, metallicity measurement and extinctiondetermination. For the first time, then, Gaia will provide us with a 3Dspatial/properties map and at least a 2D velocity map of these tracers (RVswill be obtained too for brighter stars.) This will be a goldmine ofinformation from which to learn about the origin and evolution of the Galacticdisk. I briefly review the Gaia mission, and then show how the expectedastrometric accuracies translate into distance and velocity accuracies andstatistics. I examine the impact Gaia should have on a few scientific areasrelevant to the Galactic disk. I discuss how a better determination of thespiral arm locations and pattern speed, plus a better reconstruction of theSun's orbit over the past billion years (from integration through theGaia-measured gravitational potential) will allow us to assess the possiblerole of spiral arm crossings in ice ages and mass extinctions on the Earth.

What will Gaia tell us about the Galactic disk?