  The Gaia mission is described, focussing on those technical aspects that are
necessary to understand the details of its external (absolute) flux
calibration. On board of Gaia there will be two (spectro)photometers, the blue
one (BP) and the red one (RP) covering the range 330-1050 nm, and the white
light (G-band) imager dedicated to astrometry. Given the fact that the focal
plane of Gaia will be constituted by 105 CCDs and the sources will cross the
the focal plane at constant speed, at different positions in each of the
foreseen passages (on average 70--80, but up to 350) in the mission lifetime,
the ``simple" problem of calibrating the integrated BP/RP and G-band magnitudes
and the low resolution BP/RP spectra flux turns into a very delicate and
complicated issue, including CTI effects, LSF variations across the focal plane
and with time, CCD gating to avoid saturation and the like. The calibration
model requires a carefully selected set of $\simeq$200 SpectroPhotometric
Standard Stars (SPSS) with a nominal precision of a few \%, with respect to
Vega.
