The Rosetta Lander is designed to land a payload of about 20 kg on the nucleus of comet Wirtanen in 2012 at 3 AU from the Sun, to perform a highly complex and integrated program of measurements immediately after landing, then following the comet evolution as it will approach perihelion. 9 sets of instruments are dedicated to analyse the elemental, isotopic, molecular & mineralogical composition of surface and sub-surface samples, to image the drilled samples and the landing site, to characterize the physical properties of the surface material, to measure the plasma around the comet and to sound its interior. The Lander and its payload are developped as an integrated international venture.