I have understood the title of this talk given to me by Dr. Westerhout to imply a review of our present ability to provide new radial velocity measurements for the many stars to be observed by the Space Astrometry projects, as well as an indication of the technical developments still required. And indeed, the quantity and quality of the existing radial velocity determinations are such that only a small fraction of the data needed can be considered available.

The challenges presented by the two projects, the Astrometry Satellite and the Space Telescope, are somewhat different, corresponding to the complementary nature of the two programmes: The dedicated Astrometry Satellite will observe a very large number of relatively bright stars, while the Space Telescope is expected to give astrometric results for much fainter, but also far fewer objects (estimated currently at perhaps 20 per year). The two programmes will therefore be considered separately.