  The observation of high-energy extraterrestrial neutrinos is one of the most
promising future options to increase our knowledge on non-thermal processes in
the universe. Neutrinos are e.g. unavoidably produced in environments where
high-energy hadrons collide; in particular this almost certainly must be true
in the astrophysical accelerators of cosmic rays, which thus could be
identified unambiguously by sky observations in "neutrino light". On the one
hand, neutrinos are ideal messengers for astrophysical observations since they
are not deflected by electromagnetic fields and interact so weakly that they
are able to escape even from very dense production regions and traverse large
distances in the universe without attenuation. On the other hand, their weak
interaction poses a significant problem for detecting neutrinos. Huge target
masses up to gigatons must be employed, requiring to instrument natural
abundances of media such as sea water or antarctic ice. The first generation of
such neutrino telescopes is taking data or will do so in the near future, while
the second-generation projects with cubic-kilometre size is under construction
or being prepared. This report focuses on status and prospects of current
(ANTARES, NEMO, NESTOR) and future (KM3NeT) neutrino telescope projects in the
Mediterranean Sea.
