Physics and Astrophysics of Ultrahigh Energy Cosmic Rays

The origin and nature of particles at the high end of the cosmic ray spectrum is unknown. This mystery attracts increasing attention since anticipated experiments such as the international Pierre Auger Project promise a strongly growing amount of data in the near future. Detailed testable predictions within theoretical scenarios therefore increasingly employ computational methods, for example, to model cosmic ray propagation. Proposed source mechanisms range from conventional shock acceleration to the decay of supermassive Grand Unification scale particles released from topological defects. These scenarios often link several subfields, from the astrophysics of gamma rays, neutrinos, and cosmic magnetic fields to the theory of particle interactions beyond accelerator energies. We present an overview of this quickly growing field.