  We introduce neutrino astronomy from 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. We review the facts
as well as the speculations about the sources. Among these gamma ray bursts and
active galaxies represent well-motivated speculations because these are also
the sources of the highest energy gamma rays, with emission observed up to 20
TeV, possibly higher.
  We discuss why cosmic accelerators are also expected to be cosmic beam dumps
producing high-energy neutrino beams associated with the highest energy cosmic
rays. Cosmic ray sources may produce neutrinos from MeV to EeV energy by a
variety of mechanisms. 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. The technology for commissioning such instruments
exists.
