Auger: What, why and how?

The Pierre Auger Observatories for the highest energy cosmic rays will be hybrid systems, employing water Cerenkov ground arrays over-looked by atmospheric fluorescence detectors. This will allow accurate reconstruction of primary cosmic ray energy and arrival direction in a cost effective manner, and without dependence on uncertain models. In this paper the design of the proposed instrument is reviewed, together with the simulations on which it is based, and the resulting performance predictions.