A few years ago, extreme environments were defined as having one or more environmental parameters showing values permanently or periodically close to the limits known for life in its various forms; only specialised organisms are able to cope with such extreme environments. Such environments can be considered as end-members of a continuum of environmental conditions that constitute limits for life as we know it. Extreme environments have been identified in marine and terrestrial biomes across the globe (deep sea, hydrothermal vents, continental margins, polar regions, hot springs, high altitude and glaciers, hot arid regions and deserts, acidic and alkaline environments, continental and seafloor subsurfaces, intertidal coastal areas, hypersaline environments, atmosphere). Outer space is also an extreme environment, comprising planetary bodies, space vessels and space itself.