GAIA spectroscopic and radial velocity performancies are reviewed on the base of ground-based test observations and simulations. The prospects for accurate analysis of stellar atmospheres (temperature, gravity, chemical abundances, rotation, peculiarities) and precise radial velocities (single stars, binaries, pulsating stars) are colorful provided the spectral dispersion is high enough. A higher dispersions also favors a given precision of radial velocities to be reached at fainter magnitudes: for example, with current parameters for GAIA spectrograph, a 1 km/sec accuracy on epoch RVs of a K0 star is reached at V~13.0 mag with 0.25 Ang/pix dispersion spectra, at V~10.3 mag for 0.5 Ang/pix, and V~6.7 mag for 1 Ang/pix. GAIA radial velocities for single stars can match the ~0.5 km/sec mean accuracy of tangential motions at V=15 mag, provided the observations are performed at a dispersion not less than 0.5 Ang/pix.

proceedings of Les Houches 2001 summer school "GAIA, an European Space Project", published by Editions De Physique, 14 pages