Mechanisms and sites of ultra high energy cosmic ray origin

We shortly discuss several astrophysical scenarios leading to cosmic ray acceleration up to extremely high energies reaching the scale of 10^{20} eV. The processes suggested in the literature include acceleration at relativistic jet terminal shocks and shear boundary layers, shocks in large scale accretion flows onto supergalactic cosmic structures, particle reflections from ultra-relativistic shocks postulated to exist in sources of gamma ray bursts, the processes involving the neutron star rotating magnetospheres and dormant quasars. Some of these objects can explain cosmic rays with highest energies if one tunes the model parameters to limits enabling the highest acceleration efficiency. We also note that some of the considered processes allow for acceleration efficiency in the Hillas diagram, beta, to be much larger than unity. The present paper is based on a review talk presented during the European Cosmic Ray Symposium in Lodz (2000).