Résumé
After general considerations on the use of model atmospheres and spectra, with special attention to the expected GAIA spectral harvest, I expose the general characteristics of classical line-blanketed model atmospheres (1D, LTE, static). The history and current status of MARCS model atmospheres, initially developed by Gustafsson et al. (1975), are then briefly described, with a few recent achievements in photometry and spectroscopy of cool stars. It seems that the MARCS models have reached their ultimate level of sophistication, within their set of limiting hypotheses. Further progress require more elaborate modeling, including non-LTE, and hydrodynamics. Synthetic spectra of M and S stars calculated for the GAIA spectral range are shown and discussed. I demonstrate that important work is necessary on the existing line lists before we can reliably extract stellar parameters from GAIA spectra.