Most stars are members of binary or multiple systems.
A large part of the available information on binary stars comes from ground-based astrometric surveys made by visual observers during the 19 th and 20 th centuries, most of which were based in the northern hemisphere. Unseen companions of astrometric binaries as well as wide common proper motion pairs were thus detected. The first high-precision search on a large scale and of the complete sky was obtained by the space astrometric mission Hipparcos. From a systematic monitoring of a sample of 118.000 stars over 3 years, some 3000 newly resolved doubles and several thousands of suspected doubles have been detected. The results of the Hipparcos mission illustrate the impact of a systematic survey, even though quite limited in time.