The observing programme, by the nature of the mission concept and the data reduction procedures outlined below, had to be pre-selected and finalised before launch. The INCA Consortium was established in 1981, and was assigned the responsibility, by the European Space Agency, of dealing with the definition of, and preparation of, all aspects of the Hipparcos observing programme. The final observing programme consists of more than 118000 stars, mostly within our Galaxy but some within the Magellanic Clouds, one quasar (3C273) and, within our solar system, 48 minor planets and three satellites of major planets (Europa, Iapetus and Titan). These selected objects were based upon 214 scientific proposals submitted to ESA in 1982, by the world-wide astronomical community, in response to an Invitation for Proposals. The selection of objects to be retained for observation according to their scientific interest was made by an ad hoc `Hipparcos Scientific Selection Committee'. Their recommendations, constrained by the satellite observing capabilities, were implemented by the INCA Consortium, in consultation with the original proposers and representatives of the data reduction and satellite project teams. During this work and with the agreement of the Hipparcos Scientific Selection Committee, additional proposals were defined by the INCA Consortium in order to optimise the scientific return expected for some specific topics, notably the introduction of the magnitude-limited survey, the selection of stars in the Magellanic Clouds and in galactic open clusters, and the introduction of stars associated with the improvement of the link to an extragalactic reference system.