  This is a review of neutrino astronomy anchored to the observational fact
that Nature accelerates protons and photons to energies in excess of $10^{20}$
and $10^{13}$ eV, respectively.
  Although the discovery of cosmic rays dates back close to a century, we do
not know how and where they are accelerated. Basic elementary-particle physics
dictates a universal upper limit on their energy of $5\times10^{19}$ eV, the
so-called Greisen-Kuzmin-Zatsepin cutoff; however, particles in excess of this
energy have been observed by all experiments, adding one more puzzle to the
cosmic ray mystery. Mystery is fertile ground for progress: we will review the
facts as well as the speculations about the sources including gamma ray bursts,
blazars and top-down scenarios.
  The important conclusion is that, independently of the specific blueprint of
the source, it takes a kilometer-scale neutrino observatory to detect the
neutrino beam associated with the highest energy cosmic rays and gamma rays. We
also briefly review the ongoing efforts to commission such instrumentation.
