  The topological defect scenario of origin of the observed highest energy
cosmic rays is reviewed. Under a variety of circumstances, topological defects
formed in the early Universe can be sources of very massive particles in the
Universe today. The decay products of these massive particles may be
responsible for the observed highest energy cosmic ray particles above
$10^{20}$ eV. Some massive particle production processes involving cosmic
strings and magnetic monopoles are discussed. We also discuss the implications
of results of certain recent numerical simulations of evolution of cosmic
strings. These results (which remain to be confirmed by independent
simulations) seem to show that massive particle production may be a generic
feature of cosmic strings, which would make cosmic strings an inevitable source
of extremely high energy cosmic rays with potentially detectable flux. At the
same time, cosmic strings are severely constrained by the observed cosmic ray
flux above $10^{20}$ eV, if massive particle radiation is the dominant energy
loss mechanism for cosmic strings.
