Cosmic-ray physics and the Pierre Auger Observatory

One of the foremost issues in astrophysics today is that of the origin of ultrahigh- energy cosmic rays. The Pierre Auger Observatory is a broadly based international effort to make a high-statistics study of the upper-end of the cosmic-ray spectrum. Auger is the rst experiment designed to work in a hybrid detection mode. It consists of an array of 1600 particle detectors spread over 3000 km2 and four uorescence telescopes placed on the boundaries of the surface array. Recent measurements from Auger of the energy spectrum and mass composition above 1018 eV are described.