  The origin of highest energy cosmic rays (UHECR) is yet unknown. In order to
understand their propagation we determine the probability that an ultrahigh
energy (above 5\cdot 10^{19} eV) proton created at a distance r with energy E
arrives at earth above a threshold E_c. The clustering of ultrahigh energy
cosmic rays suggests that they might be emitted by compact sources. A
statistical analysis on the source density based on the multiplicities is
presented. The ultrahigh energy cosmic ray spectrum is consistent with the
decay of GUT scale particles. Alternatively, we consider the possibility that a
large fraction of the ultrahigh energy cosmic rays are decay products of Z
bosons which were produced in the scattering of ultrahigh energy cosmic
neutrinos on cosmological relic neutrinos. Based on this scenario we determine
the required mass of the heaviest relic neutrino. The required ultrahigh energy
neutrino flux should be detected in the near future by experiments such as
AMANDA, RICE or the Pierre Auger Observatory.
