The energy spectrum of cosmic rays extends to beyond 10²⁰ eV, i.e., to energies higher by eight orders of magnitude than the highest energies attained so far at terrestrial particle accelerators. The origin of these particles remains unknown: neither their sources, nor the underlying physical processes of emission and acceleration of them have been identified. Although many models of cosmic ray origin have been proposed, a conclusive verification of them has not been possible so far due to limited statistics of available experimental data on the cosmic ray energy spectrum and composition at the highest energies. The current experimental data are reviewed in this paper. Prospects to gather sufficiently large statistics in the near future, especially from the Pierre Auger Observatory, are discussed.

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