The Hipparcos Input Catalogue was constructed as the observing programme for the European Space Agency's Hipparcos astrometry mission. The requirements of the project in terms of completeness, sky coverage, astrometric and photometric accuracy, as well as the necessary optimisation of the scientific impact, resulted in an extended effort to compile and homogenize existing data, to clarify sources and identifications and, where needed, to collect new data matching the required accuracy.
This has resulted in an unprecedented catalogue of stellar data including up-to-date information on positions, proper motions, magnitudes and colours, and (whenever available) spectral types, radial velocities, multiplicity and variability information. The catalogue is complete to well-defined magnitude limits, and includes a substantial sampling of the most important stellar categories present in the solar neighbourhood beyond these limits. As explained below, the magnitude limits vary from 7.3 to 9 mag as a function of galactic latitude and spectral type, and there are no stars fainter than about mag.