  The origin of cosmic rays is one of the major unresolved questions in
astrophysics. In particular, the highest energy cosmic rays observed possess
macroscopic energies and their origin is likely associated with the most
energetic processes in the Universe. They thus provide a probe of physics and
astrophysics at energies that are unreached in laboratory experiments.
Theoretical explanations range from acceleration of charged particles in
astrophysical environments to particle physics beyond the well established
Standard Model, and processes taking place at the earliest moments of our
Universe. Distinguishing between these scenarios requires detectors with
effective areas in the 1000 km^2 range which are now under construction or in
the planning stage. Close connections with gamma-ray and neutrino astrophysics
add to the interdisciplinary character of this field.
