From the Galaxy to the Edge of the Universe: Plausible Sources of UHECRs
  The lack of a high energy cutoff in the cosmic ray spectrum together with an apparently isotropic distribution of arrival directions for the highest energy events have strongly constrained most models proposed for the generation of these particles. An overview of the theoretical proposals are presented along with their most general signatures. Future experimental tests of the different proposals are discussed. 
Angela V. Olinto