In this review the origin of cosmic rays (CR) at ultra high energies (UHE) E > 4 * 10¹⁹ eV is discussed. The hypothesis of CR acceleration in active galactic nuclei (AGN) is mostly eliminated. The review contains five sections. In Sec. 1 the following points are considered: arrays for detecting UHECRs, the problem of GZK cut-off, and ideas for UHECR origin. Some astrophysical data that are used in CR physics are given in the special subsection as CR are of extraterrestial origin. In Sec. 2, results of identification of UHECR sources are considered. CR acceleration in the identified sources and CR escaping from sources are treated in Sec. 3. In Sec. 4, CR propagation in extragalactic space, namely energy losses due to interactions with cosmic background radiation and due to Universe expansion, are considered, and the spectrum of CR protons at the Earth is calculated. In Sec. 5 results are formulated. Further analysis of the hypothesis includes calculation of propagation of CR nuclei and investigation of UHECR composition at the Earth. Because of this nuclear physics data which are required for solving this problem are listed in the conclusion.

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