  The origin of the highest-energy cosmic rays remains a mystery. The lack of a
high energy cutoff in the cosmic ray spectrum together with an apparently
isotropic distribution of arrival directions have strongly constrained most
models proposed for the generation of these particles. An overview of the
present state of theoretical proposals is presented. Astrophysical accelerators
as well as top-down scenarios are reviewed along with their most general
signatures. The origin and nature of these ultra-high energy particles will be
tested by future observations and may indicate as well as constrain physics
beyond the standard model of particle physics.
