  The existence of cosmic rays of energies exceeding 10^20 eV is one of the
mysteries of high energy astrophysics. The spectrum and the high energy to
which it extends rule out almost all suggested source models. The challenges
posed by observations to models for the origin of high energy cosmic rays are
reviewed, and the implications of recent new experimental results are
discussed. Large area high energy cosmic ray detectors and large volume high
energy neutrino detectors currently under construction may resolve the high
energy cosmic ray puzzle, and shed light on the identity and physics of the
most powerful accelerators in the universe.
