  Several high energy, >100 GeV, neutrino telescopes are currently operating or
under construction. Their main motivation is the extension of the horizon of
neutrino astronomy to cosmological scales. We show that general, model
independent, arguments imply that ~1 Gton detectors are required to detect
cosmic high energy neutrino sources. Predictions of models of some of the
leading candidate sources, gamma-ray bursts and micro-quasars, are discussed,
and the question of what can be learned from neutrino observations is
addressed.
