The vast volume and expanse that the deep-sea in general represents a challenging environment to colonize. Pressure is the most basic of these challenges but the cold temperature, absence of light, food scarcity, and sometimes limited oxygen supply have been the focus of many studies over the years. Since the discovery of deep-sea hydrothermal vents in the late 1970’s our perception of the deep-sea has changed. In these habitats, food scarcity is no longer an issue because of the local primary production by microbes that use reduced chemicals contained in the vent fluid (chemosynthesis). For metazoans, taking advantage of this food source, however, requires specific adaptation to deal with the harsh conditions that are encountered near hydrothermal vents. In particular, the chaotic mixing of the hydrothermal fluid with the cold deep-sea water creates highly variable -and unpredictable- conditions. For these animals, temperature and low oxygen concentrations are probably the first constraints to deal with. The hydrothermal fluid also contains compounds that are toxic (heavy metals, H2S, …), and its pH is very low compared to seawater. All these challenges require specific adaptations that are found in vent species. This explains at least in part the fact that 95% of the species are endemic of this type of environment.