  The construction of large volume detectors of high energy, >1 TeV, neutrinos
is mainly driven by the search for extra-Galactic neutrino sources. The
existence of such sources is implied by observations of ultra-high energy,
>10^{19} eV, cosmic-rays, the origin of which is a mystery. In this lecture I
briefly discuss the expected extra-Galactic neutrino signal and the current
state of the experimental efforts. Neutrino emission from gamma-ray bursts
(GRBs), which are likely sources of both high energy protons and neutrinos, is
discussed in some detail. The detection of the predicted GRB neutrino signal,
which may become possible in the coming few years, will allow one to identify
the sources of ultra-high energy cosmic-rays and to resolve open questions
related to the underlying physics of GRB models. Moreover, detection of GRB
neutrinos will allow one to test for neutrino properties (e.g. flavor
oscillations and coupling to gravity) with an accuracy many orders of magnitude
better than is currently possible.
