  UHE neutrinos with $E>10^{17}$ eV can be produced by ultra-high energy cosmic
rays (UHECR) interacting with CMB photons (cosmogenic neutrinos) and by
top-down sources, such as topological defects (TD), superheavy dark matter
(SHDM) and mirror matter. Cosmogenic neutrinos are reliably predicted and their
fluxes can be numerically evaluated using the observed flux of UHECR. The lower
limit for the flux is obtained for the case of pure proton composition of the
observed UHECR. The rigorous upper limit for cosmogenic neutrino flux also
exists. The maximum neutrino energy is determined by maximum energy of
acceleration, which at least for the shock acceleration is expected not to
exceed $10^{21} - 10^{22}$ eV. The top-down sources provide neutrino energies a
few orders of magnitude higher, and this can be considered as a signature of
these models. Oscillations play important role in UHE neutrino astronomy. At
production of cosmogenic neutrinos $\tau$-neutrinos are absent and
$\bar{\nu}_e$ neutrinos are suppressed. These species, important for detection,
appear in the observed fluxes due to oscillation. Mirror neutrinos cannot be
observed directly, but due to oscillations to ordinary neutrinos they can
provide the largest neutrino flux at the highest energies.
