Crustaceans have successfully colonized the subterranean habitat, and many have become obligate inhabitants, occurring virtually everywhere there are interconnected voids underground. With the exception of most oniscidean isopods and a few talitrid amphipods, subterranean crustaceans inhabit water (stygobionts), where they dominate the stygofauna both in biomass and diversity of species. Four major taxonomic groups predominate: amphipods, isopods, copepods, and ostracods. Although most higher crustacean taxa have representatives in both epigean and subterranean habitats, some groups such as remipedes, thermosbaenaceans, spelaeogriphaceans, bathynellaceans, mystacocarids, and gelyelloid copepods are known only from the subterranean environment. Subterranean habitats vary physically and range from organically rich shallow habitats around seeps to cave systems more than a kilometer deep. Water quality, which can range from fresh to marine to hypersaline, static to flowing, and oxic to suboxic, impacts species distribution. Dispersal patterns in subterranean crustaceans are also diverse. Freshwater stygobiont crustaceans have narrow endemic ranges, and their dispersal is limited by saltwater. The distribution of several freshwater taxa might reflect the movement of tectonic plates. The extraordinarily diverse anchialine fauna, initially distributed along the Tethyan coast, was likely spread by vicariance due to movement of tectonic plates. Originating from epigean ancestors, many stygobionts have a marine origin. While the existence of preadaptations does not necessarily guarantee successful colonization of the subterranean habitat, a suite of characteristics is frequently observed in subterranean crustaceans, with most being weakly chitinized, lacking or with reduced eyes and pigments, and enhanced non-optic sense organs. Metabolic rates tend to be lower than in epigean crustaceans. Limited evidence indicates subterranean crustaceans are longer lived with lower reproductive potential. These adaptations make subterranean crustacean populations particularly vulnerable to anthropogenic impacts. The morphological, physiological, and life history adaptations to a subterranean existence are most likely common responses to the physical environment of each subterranean ecosystem. Extensive biodiversity and phylogeography studies are still required, and there is a pressing need to comprehend the functional role of stygofauna in subterranean waters.