Astronomical and Astrophysical Objectives of Sub-Milliarcsecond Optical Astrometry contains a series of papers that discuss the major changes occurring in modern astrometry. The results of space missions in astrometry, Hipparcos and some results from the Hubble Space telescope are presented for the first time. Combined with ground based results, this provides astrometry results at milliarcsecond resolution. At the same time, the extragalactic reference frame, based on very long baseline interferometry radio positions, is being introduced as the fundamental reference frame. It is now also evident that future optical interferometry space missions can provide an additional improvement in future of orders of magnitude. In addition to presenting the results, the book also discusses different applications based on such accurate astrometric positions. This startling improvement in resolution will encourage all astronomers to consider what can be done in future, providing a great stimulus to forthcoming research.