Many kinds of radiation exist in the universe, including photons and particles with a wide range of energies. Some of the radiation is produced in stars and galaxies, and some is cosmological background radiation, a relic from the history of cosmic evolution. Among all this radiation, the most energetic are cosmic ray particles: nucleons, nuclei, and even extremely energetic gamma rays. There are some observational facts about cosmic rays to give suggestions on their origin. The most important one among them is that the energy spectrum of high energy cosmic rays above 10 GeV (where the magnetic field of the sun is no longer a concern) is well represented by a power law form. This indicates cosmic ray particles are products of non-thermal processes. Their energy extends over more than 13 decades from 107 eV up to 1020 eV. In terms of its structure, the spectrum can be divided into three regions: two 'knees' and one 'ankle'. The first 'knee' appears around 31015 eV where the spectral power law index changes from -2.7 to -3.0. The second 'knee' is somewhere between 1017 eV and 1018 eV where the spectral slope changes from -3.0 to around -3.3. The 'ankle' is seen at or after 31018 eV. Above that energy the spectral slope is around -2.7, but with a large uncertainty because of poor statistics and resolution. This book deals with the final and most energetic population, the Ultra High Energy Cosmic Rays (UHECRs).