Ultra high energy cosmic rays

The evidence for the existence of cosmic rays with energies in excess of 10(20) eV is now overwhelming. There is so far no indication of the GZK cutoff in the energy spectrum at 5 x 10(19) eV. This conclusion is not firm for lack of statistics. A cutoff would be expected if the sources of the cosmic rays were distributed uniformly throughout the cosmos. The sources of cosmic rays with energy above the GZK cutoff must be at a distance less than or equal to 100 Mpc, and if they are protons they are very likely to point to these sources. There are no easy explanations how known astrophysical objects can accelerate protons (or atomic nuclei) to these energies. The fluxes of these cosmic rays is very low and large instruments are required to observe them even with modest statistics. One such instrument, the Pierre Auger Observatory, is described. It is designed for all-sky coverage and the construction of its southern site in Argentina has begun.