Subject : By 2017, the Gaia mission will provide astrometric solutions (parallaxes, proper motions) for most of the sky, as well as radial velocities for the brightest stars and related astrophysical parameters. This data, complemented by follow-up spectroscopic surveys (RAVE, GES, APOGEE, GALAH), will allow to derive the essential of stellar motions, chemo-dynamical relations, and ages for hundred thousands of stars. Thus, without waiting for the publication of the final Gaia catalogue (2022), the next years will already provide an unique ensemble of observational data to interpret. The interested student will start to learn how to make use of the tremendous amount of data Gaia will deliver to reconstruct and interpret stellar motions. The aim of this work, in particular, is to analyze high-resolution N-body simulations of a Milky Way-type galaxy accreting one or several satellites to quantify the evolution of the disc structural properties (scale height of old and young stellar populations, radial migration of stars). The ultimate goal is to set a limit to the amount of accreted mass by mergers that the Galaxy has experienced in the last 10 Gyr.