Opening the Ultra High Energy Cosmic Ray window from the top

While several arguments can be proposed against the existence of particles with energy in excess of $(3-5)\times 10^{19}$ eV in the cosmic ray spectrum, these particles are actually observed and their origin seeks for an explanation. After a description of the problems encountered in explaining these ultra-high energy cosmic rays (UHECRs) in the context of astrophysical sources, we will review the so-called {\it Top-Down} (TD) Models, in which UHECRs are the result of the decay of very massive unstable particles, possibly created in the Early Universe. Particular emphasis will be given to the signatures of the TD models, likely to be accessible to upcoming experiments like Auger.