Thepublication of theHipparcos andTycho Catalogues in 1997 transformed astrometry, and as a consequence astronomers’ perception of astrometry. What had before often been regarded as a somewhat quaint specialty of limited re- vance to modern astrophysics, was suddenly seen to produce a wealth of data of immediate practical use. The ready availability of many thousand precise trigonometric stellar distances and the access to an accurate and dense - tical reference frame have changed the way astronomers think about certain problems and plan their experiments. Inevitably, the exploitation of so much new data not only solved some old problems, tidied up several confused areas and sharpened many observational constraints, but it also generated new qu- tions about established theory – and about the data themselves. The author of this book has taken a radical approach to answer some of these questions: a complete re-examination of the satellite data and the models used to represent them, in particular the attitude modelling. Eventually this resulted in the new and very significantly improved Hipparcos reduction described in this book. This remarkable achievement was made possible by a combination of many factors, including time and the exponential growth of computing power, but mainly an incredible amount of detailed, tedious and ingenious work by the author and the resulting insight into what really went on with the satellite in its unhappy orbit. Dr.