The Hipparcos Input Catalogue has been compiled, over the period 1982-1991, as the definitive observing catalogue for the European Space Agency’s Hipparcos satellite, launched on 8 August 1989. It contains the most up-to-date, comprehensive and homogeneous information on the 118 000 stars being observed by Hipparcos. Its stellar and data content is described in a series of three papers. Details of the astrometric data are presented in Paper II, and of the photometric data (including photoelectric standards stars) in Paper III. The present paper deals with the stellar content of the catalogue and the way it was constructed.