  Existing and planned neutrino detectors, sensitive in the energy regime from
10^{17} eV to 10^{23} eV, offer opportunities for particle physics and
cosmology. In this contribution, we discuss particularly the possibilities to
infer information about physics beyond the Standard Model at center-of-mass
energies beyond the reach of the Large Hadron Collider, as well as to detect
big bang relic neutrinos via absorption dips in the observed neutrino spectra.
