A vast ground-based observational effort was launched several years ago in order to prepare the optimal selection of targets for the COROT seismology programme. The first objective of this ground-based programme was to characterize as completely as possible all potential targets of the mission, in particular to determine their effective temperature, surface gravity, metallicity, and projected rotation velocity. In order to achieve this goal, photometric and high resolution spectroscopic observations were obtained for more than 2,000 stars, and made available to the community via the Internet in the GAUDI database. Moreover, a systematic search for classical pulsators, including δ Sct, γ Dor, β Cep, Be stars, in the COROT eyes, and their detailed characterization, was undertaken in order to optimize the selection of such stars in the COROT seismology programme. This involved photometric monitoring and high resolution spectroscopy of several dozen stars, from which the fundamental parameters and pulsational properties of these potential COROT targets were determined. Additional preparatory ground-based observations were also carried out, including Ca II H&K monitoring for studying magnetic activity of some of the COROT potential targets, and adaptive optics high angular resolution imaging for characterizing the nearby fields around the targets. We are now planning ground-based observations in support and as a follow-up of the COROT seismology programme. These include high resolution spectroscopic monitoring of classical pulsators observed by COROT in order to complement the COROT photometric data with spectroscopic data giving access to high degree modes; spectropolarimetric observations for detecting and mapping magnetic fields at the surface of cool stars among the COROT targets; and interferometric measurements of the diameters of COROT targets.