Gaia is an ESA's cornerstone mission scheduled for launch in 2012. Its goal is to to chart a three-dimensional map of our Galaxy, the Milky Way, in the process revealing the composition, formation and evolution of the Galaxy. Gaia will provide unprecedented positional and radial velocity measurements with the accuracies needed to produce a stereoscopic and kinematic census of about o ne billion stars in our Galaxy and throughout the Local Group.To achieve these ambitious goals the raw data of the satellite has to be submitted to a complex data processing that transforms the instrument data into scientific data. The data processing system will be run in five separated data centers, with a central hub at ESAC. The building and testing of such a complex and distributed system is a very challenging task, and to carry it out realistic simulations of the Gaia data are needed. Our group at the Departament d'Astronomia i Meteorologia is in charge of these simulations and during the talk I will describe the design of the Gaia simulator, how it works and present some examples of its outputs.