The goal of the Hipparcos mission is an ambitious one - the creation of an astrometric catalogue of some 120 000 stars,
with an unprecedented precision on each of their five astrometric parameters. With these data, the astronomical community will be provided with fresh and dramatic insights into many aspects of this fundamental branch of science. Astrometrists will have at their disposal a vast quantity of accurate and homogeneous data which can be used to improve upon the present knowledge of the optical reference frame, and which will provide a dense reference system for past and future ground-based observations. Astrophysicists will be provided with parallaxes and proper motions of not only a highly accurate but also a remarkably homogeneous nature. Important
by-products of the measurements will be a systematic search for binary and multiple star systems, and about 100 high-precision photometric measurements for each star distributed over the
2.5-year mission lifetime. This paper provides an outline of the principals of the satellite operation, the predicted accuracy status,
an overview of the Input Catalogue contents, and a summary of the steps constituting the data reduction. Special emphasis is given to the activities ensuring that a single Hipparcos Catalogue emerges from the parallel work of the two data analysis teams.