Large-volume neutrino observatories based on monolithic liquid targets are discussed, as they are clearly able to address key issues of particle and astro-particle physics by extrapolating existing neutrino detection technologies. There is a growing effort in the US, Europe and Japan for the realization of such projects. Three detector technologies are under discussion: water Cherenkov detectors, liquid argon Time Projection chambers, and a liquid scintillator-based detector. A general overview on the motivation, some selected examples for the physics potential, and a short discussion of the status of various projects is given.