Very Long Baseline Interferometry - a high precision tool for geodesy and astrometry

Very Long Baseline Interferometry is able to provide a direct geometrical tie to the extragalactic radio sources which represent the best possible realization of an inertial system. By this token, VLBI can measure Earth rotation and orientation as well as precise station positions and their velocities without involving the gravity field of the Earth. In the broader context of Earth observation and the monitoring of geodynamic processes, this one and many more unique features have allowed the VLBI technique to achieve pioneering feats such as the determination of present-day plate tectonic motions, post glacial rebound, sub-daily Earth rotation variations and parameters of general relativity. In this contribution, a brief outline of the VLBI technique and the models used in geodetic data analysis will be given before some of the most important achievements to date will be passed in review and future developments will be indicated. (C) 2000 Academie des sciences/Editions scientifiques et medicales Elsevier SAS.