Deep-water habitats form by far the largest ecosystems on the planet. Enough is known about the lives of deep-water fishes, despite the difficulties inherent in such study, to give a fairly coherent picture of their overall ecology. Inferences based on individual morphology and taxonomic diversity provided the first clues, but series from the various large national deep-water expeditions have allowed the working out of detailed life histories, food webs, species associations and biogeography. The particular conditions that characterize deep water everywhere have shaped the nature of deep-water fishes, and probably the most important factor at play has been the logarithmic decline of food energy available with increasing depth. Comparative study of deep-pelagic, deep-demersal, and deep-lake fishes reveals interesting and intriguing differences with respect to the adaptations and probable history of the faunas in each of these habitats. Deep-water fishes have long been insulated from any significant human impacts, but deep-water fisheries, species introductions, pollution and other ecosystem tamperings are changing this situation rapidly. Can these remarkable, enduring and well-adapted species and assemblages withstand the developing onslaught?