The existence of cosmic-rays of energies exceeding 1020 eV is one of the great mysteries of high energy astrophysics. Recent new experimental results, and their implications to theoretical models, will be reviewed. It will be shown that model predictions may be tested with high energy gamma-ray, cosmic-ray and neutrino detectors under construction. Combined gamma-ray, neutrino and cosmic-ray observations may resolve the high energy cosmic ray puzzle and will shed light on the nature and physics of the most powerful accelerators in the universe.