De la recherche du bien

The following topics are addressed: space astrometric missions, ground based optical techniques, very long baseline interferometric (VLBI) techniques, impact of these techniques on astrophysics and on geodynamics, and issues related to celestial and terrestrial reference frames. In the area of space optical astrometry, the reduction of one year of data from the Hipparcos mission shows excellent results for more than 40,000 stars. Recent developments in ground-based optical astrometry are concentrated on CCB meridian circles and transit instruments. The VLBI technique is the most powerful method to determine the earth rotation parameters, nutation series, and baseline lengths. More accurate celestial and terrestrial reference frames are needed and recent achievements in this field are presented, for example FK5 catalogues and a comparison of VLA and Hipparcos positions.