  Measurements of the arrival directions of cosmic rays have not revealed their
sources. High energy neutrino telescopes attempt to resolve the problem by
detecting neutrinos whose directions are not scrambled by magnetic fields. The
key issue is whether the neutrino flux produced in cosmic ray accelerators is
detectable. It is believed that the answer is affirmative, both for the
galactic and extragalactic sources, provided the detector has kilometer-scale
dimensions. We revisit the case for kilometer-scale neutrino detectors in a
model-independent way by focussing on the energetics of the sources. The real
breakthrough though has not been on the theory but on the technology front: the
considerable technical hurdles to build such detectors have been overcome.
  Where extragalactic cosmic rays are concerned an alternative method to probe
the accelerators consists in studying the arrival directions of neutrinos
produced in interactions with the microwave background near the source, i.e.
within a GZK radius. Their flux is calculable within large ambiguities but, in
any case, low. It is therefore likely that detectors that are larger yet by
several orders of magnitudes are required. These exploit novel techniques, such
as detecting the secondary radiation at radio wavelengths emitted by neutrino
induced showers.
