The Pierre Auger Observatory is an international facility dedicated to the full-sky study of the highest-energy cosmic rays. The southern site of the Auger Observatory was completed in June 2008. Data collected since January 2004 have yielded important information on the energy spectrum, the primary particle composition, the fluxes of photons and neutrinos and on the anisotropic distribution of the arrival directions of the most energetic particles. On this basis, the scientific motivation for the northern Auger Observatory site in Colorado, USA, is discussed. The overall layout, the key components and the expected performance of this 20000km 2 hybrid observatory comprised of an array of 4400 surface detectors and 39 fluorescence telescopes are described.