Gaia is a unique ESA astronomical satellite nicknamed "The billion-star surveyor" as it will deliver positions, motions, distances, photometry, and possible binary nature of 2 billion stars of our Galaxy. This European endeavour has been made possible thanks to the joint efforts of about 250 astronomers across Europe for 15 years, with a strong participation of Belgium. ULB Institut d'Astronomie et d'Astrophysique is leading the work on non-simple objects, investigating all objects for which a single-star solution appears unsatisfactory.