Two types of accurate astrometric data are considered: 1. 1) Survey astrometry providing absolute parallaxes and annual proper motions with an accuracy of 2 milli-arcsec, to a few tenth of a milli-arcsec. Parallax determinations will advance our basic knowledge of the Universe by giving the necessary strong foundations for the first steps of the cosmic distance scale. It will make available absolute luminosities and masses of an extended variety of stellar types in different evolutionary stages, necessary for a better understanding of stellar structure and evolution. Absolute proper motions in a consistent reference frame will drastically improve our insight of stellar populations, and thereby of galactic structure, age and evolution. 2. 2) Local astrometry measuring apparent relative displacements of a small group of stars with accuracies that may reach 0.01 and possibly 0.001 milli-arcsec. The expected results are an increase of the number of accurately known stellar masses, an analysis of relativistic effects in stellar environment, the discovery of unseen companions of stars, and internal motions in the Magellanic Clouds.