  In this paper we review the hypothesis that a considerable part of the cosmic
ray flux observed above about $10^{19}\eV$ may be produced by decaying or
annihilating topological defects left over from phase transitions in the early
universe at grand unification energy scales ($\approx10^{16}\GeV$). Possible
signatures of cosmic ray producing defect models are discussed which could be
tested experimentally in the near future. We thereby focus on model independent
universal spectral properties of the predicted particle fluxes.
