PLATO is a class-M ESA mission selected for assessment study in the Cosmic Vision context. It will characterize extrasolar planetary systems through transit detections and detailed asteroseismological stellar observations, together with follow-up spectroscopic observations. PLATO is designed to detect Earth-size (or even smaller) planets in the Habitable Zone. The large observed star sample will allow meaningful comparisons between planetary systems in different conditions (age, metallicity...) giving crucial constraints to planetary formation theories.