An extremophile (from Latin extremus meaning "extreme" and Greek philiā (φιλία) meaning "love") is an organism that thrives in and even may require physically or geochemically extreme conditions that are detrimental to the majority of life on Earth.

On Earth, most plant and animal cells thrive within a specific range of temperatures, and tend to do more poorly or die when those extremes are surpassed. Most earthlings thrive and engage in life sustaining activity within a temperature range of 0 to 48 C.. Beyond 50 degrees most living cells begin to cook. The protein molecules making up the cell's cytoplasm and membrane, begin to melt and turn into a coagulated mush. The cell dies.

However, the genomes of some microbes, including yeasts and bacteria, are able to readjust the molecular organization of their membranes and can easily cope with extremes in temperatures. They manufacture of "heat-shock" proteins which is a common biological reaction to environmental extremes. Once secreted, "heat shock" proteins wrap around the outer microbial protein layers thereby forming a protective molecular heat shield (Brock et al. 1996; Postgate, 1994). Heat shock proteins enable creatures such as thermophiles to flourish in exceedingly hot water and in boiling seas.

Consider, for example, Thermus aquaticus, which sojourns in the hot springs at Yellowstone National Park. These microbes rapidly multiply even at temperatures in excess of 80 degrees Celsius! and form huge colonies which make the water bubble in vivid hues of blues, greens and yellows--their natural colors. Normally, cells and their DNA tends to melt-down at around 75 degrees.

Even more remarkable are the ultra-thermophile microbes such as Methanopyrus which thrive near deep ocean hot vents where temperatures range from 100 to 115 degrees (Holm, 1992). These creatures obtain carbon from sea water, including atmospheric CO2 that has dissolved in sea water. They do not rely on photosynthesis as an energy source. Hence, they are also referred to as chemoautotrophs.

At the other extreme are those creatures which prefer temperatures well below freezing, and dwell hidden in rocks and crevices in frigid locals such as Antarctica. The capacity to live under these frigid conditions in made possible through the manufacture of glycerol--an organic anti-freeze. Glycerol prevents the freezing of red blood cells, semen, and complex tissues by preventing ice formation. Glycerol is also produced by a variety of insects and arthropods, as well as plant cells. Complex multi-cellular life, therefore, can live under extreme temperatures--including, as recently discovered, mites in Antarctica.

In addition to glycerol, some microbes and multi-cellular creatures manufacture an especially thick and fatty cell membrane which enables them to not only survive but to thrive under exceedingly frigid temperature and Arctic extremes. Again, consider the Antarctic mites--creatures that normally infest cats.

One species of cold loving microbe, the psychophrils (e.g. psychtrophilic algae) grows atop the snow in the Swiss Alps, usually turning pink as they are bleached in the sun. Similar creatures may in fact inhabit the Neptune's planet sized moon, Triton (Joseph, 1997), which also displays a waxing and waning pinkish coloration. Others prefer living in the bottom of the Arctic sea, munching on organic debris which drift down from the frigid upper layers of the ice bound ocean. Some micro-algae and complex multi-cellular fungi tend to congregate in the frozen tundra, deep in the permafrost, whereas others prefer Arctic lakes and will continue to multiply even at temperatures well below -30 degrees Celsius (Brock et al. 1996; Postgate, 1994). Single and complex multi-cellular life has also been found in the frozen depths of Antarctic ice, over two miles down, including algae, fungus, spores, and plants called diatoms--creatures and plants that scientists from NASA's Jet Propulsion Lab (see below) believe may have arrived here from outer space.

Even when dips in temperature exceed their general capacity for survival, those microbes and multi-cellular animals and plants who die will hemorrhage and spill out their cellular contents, including glycerol. As these creatures live in colonies, the release of these substances prevents the death of their fellows whose outer membranes are coated with the glycerol anti-freeze hemorrhaged by their dying neighbors.

Moreover, there are bacteria, such as Deinococcus radiodurans, which can withstand the effects of an atomic bomb and which greedily digest atomic waste. It can also survive high doses of ultraviolet radiation and will enter into dormancy if deprived of these delicacies and essential nutrients.

Hot and cold loving microbes, algae, fungi, mites, and primitive plants, such as those described above, and of course, Deinococcus radiodurans, are well adapted for surviving a debris encased journey through outer space.

Moreover, in addition to living on and in the Earth, these and like-minded creatures may well populate other planets, including Mars, Neptune, and the moons Europa, Titan, and Triton.

Most known extremophiles are microbes. The domain Archaea contains renowned examples, but extremophiles are present in numerous and diverse genetic lineages of both bacteria and archaeans. Furthermore, it is erroneous to use the term extremophile to encompass all archaeans, as some are mesophilic. Neither are all extremophiles unicellular; protostome animals found in similar environments include the Pompeii worm, the psychrophilic Grylloblattodea (insects), Antarctic krill (a crustacean), and the "water bear".

Acidophile: An organism with an optimum pH level at or below pH 3

Alkaliphile: An organism with optimal growth at pH levels of 9 or above

Endolith: An organism that lives in microscopic spaces within rocks, such as pores between aggregate grains; these may also be called cryptoendoliths, a term that also includes organisms populating fissures, aquifers, and faults filled with groundwater in the deep subsurface.

Halophile: An organism requiring at least 0.2M concentrations of salt (NaCl) for growth.

Hyperthermophile: An organism that can thrive at temperatures between 80–122 °C, such as those found in hydrothermal systems.

Hypolith: An organism that lives inside rocks in cold deserts.

Lithoautotroph: An organism (usually bacteria) whose sole source of carbon is carbon dioxide and exergonic inorganic oxidation (chemolithotrophs) such as Nitrosomonas europaea; these organisms are capable of deriving energy from reduced mineral compounds like pyrites, and are active in geochemical cycling and the weathering of parent bedrock to form soil.

Metalotolerant: capable of tolerating high levels of dissolved heavy metals in solution, such as copper, cadmium, arsenic, and zinc; examples include Ferroplasma sp. and Ralstonia metallidurans.

Oligotroph: An organism capable of growth in nutritionally limited environments.

Osmophile: An organism capable of growth in environments with a high sugar concentration.

Piezophile: An organism that lives optimally at high hydrostatic pressure; common in the deep terrestrial subsurface, as well as in oceanic trenches.

Polyextremophile: An organism that qualifies as an extremophile under more than one category.

Psychrophile/Cryophile: An organism that grows better at temperatures of 15 _° C or lower; common in cold soils, permafrost, polar ice, cold ocean water, and in or under alpine snowpack.

Radioresistant: Organisms resistant to high levels of ionizing radiation, most commonly ultraviolet radiation, but also including organisms capable of resisting nuclear radiation.

Thermophile: An organism that can thrive at temperatures between 60–80 _° C.

Thermoacidophile: Combination of thermophile and acidophile that prefer temperatures of 70–80 _{° Xerophile: An organism that can grow in extremely dry, desiccating conditions; this type is exemplified by the soil microbes of the Atacama Desert.}