  The Gaia mission is described, along with its scientific potential and its
updated science perfomances. Although it is often described as a
self-calibrated mission, Gaia still needs to tie part of its measurements to
external scales (or to convert them in physical units). A detailed decription
of the Gaia spectrophotometric standard stars survey is provided, along with a
short description of the Gaia calibration model. The model requires a grid of
approximately 200 stars, calibrated to a few percent with respect to Vega, and
covering different spectral types.
