  Gigantic neutrino telescopes are primarily designed to search for very high
energy neutrino radiation from the cosmos. Neutrinos travel unhindered over
cosmological distances and therefore carry unique undistorted information about
its production sites: the most powerful accelerators of hadrons in nature.
  In these lectures, we present the relevant physics motivations and their
specifics. We review methodological aspects of neutrino telescopes: the
experimental technique, some of the faced problems and the capabilities in
terms of discovery potential, effective area, isolation of a signal from
atmospheric backgrounds, etc. Instruments and their operation in various media
are described.
  We also mention the instrumental birth and provide an outlook of the
detection technique toward very low and ultra-high energies.
