Astrometry is the domain of astronomy devoted to the determination of po sitions and their time-variations, and by extension, the apparent dimensions and shapes of celestial bodies. Although several books describe the theoretical foundations of positional astronomy, they touch only slightly on the descrip tion of instruments and the procedures for obtaining actual geometrical or kinematic quantities, which are among the basic observational data in the study of the Universe and of its components. The goal of the present book is, in contrast, to provide an up-to-date description of astrometric techniques, particularly the most recent and powerful ones, whether the instruments are on the ground or in space. Until the end of the 19th century, before the development of physical as tronomy, all astronomical observations were directed towards obtaining posi tions of celestial bodies. Since then astrophysics has become the most impor tant domain of astronomy. With the extension of observations to almost all wavelengths from radio waves to gamma rays, with the use of very sensitive new receivers and the development of fast computers, remarkable progress has been made in the description and the understanding of the Universe.
