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 their 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 ultrahigh energies.