  The observation of cosmic ray particles with unexpected high energies is
pushing astroparticle physics into a period of rapid progress both
theoretically and experimentally. Different proposed models for the generation
of these particles are constrained by the absence of the predicted GZK cutoff
in the cosmic ray spectrum and by the composition and the distribution of
arrival directions observed. The database increase due to the Pierre Auger
Observatory will provide a clearer picture of the spectral anisotropy and
properties of such high energy particles, enabling tests of their origin and
nature.
