  We discuss the production of ultra high energy neutrinos coming from the
propagation of ultra high energy cosmic rays and in the framework of top-down
models for the production of these extremely energetic particles. We show the
importance of the detection of ultra high energy neutrinos that can be a
fundamental diagnostic tool to solve the discrepancy in the observed chemical
composition of ultra high energy cosmic rays and, at the extreme energies, can
unveil new physics in connection with the recent cosmological observations of
the possible presence of tensor modes in the fluctuation pattern of the cosmic
microwave background.
