An extremophile is any microbe that thrives in extreme conditions of temperature, pressure, salinity, or concentrations of hostile chemicals. Many extremophiles belong to the kingdom Archaea, also known as Archaebacteria, and most known Archaebacteria are extremophiles. There are also extremophiles to be found outside the kingdom Archaea; for example there are bacteria and eukaryotic prokaryotes that are extremophiles, as well as species of worm, crustacean, and krill.

Following are different types of extremophiles. Some extremophiles may fit into more than one category:

• 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 inside rocks.
• Halophile: An organism requiring at least 2M of NaCl for growth.
• Hyperthermophile: An organism that can thrive at temperatures between 80-110°C (176-230°F).
• Hypolith: An organism that lives inside rocks in cold deserts.
• Metalotolerant: capable of tolerating high levels of heavy metals, such as copper, cadmium, arsenic, and zinc.
• Oligotroph: An organism capable of growth in nutritionally limited environments.
• Piezophile: An organism that lives optimally at high hydrostatic pressure.
• Polyextremophile: An organism that can survive different extreme conditions.
• Psychrophile: An organism that can thrive at temperatures of 15°C (59°F) or lower.
• Radioresistant: resistant to high levels of ionizing radiation.
• Thermophile: An organism that can thrive at temperatures between 60-80°C (140-176°F).
• Xerophile: An organism that can grow in environments with a low water activity.

The Pompeii worm, an extremophile, lives on the floor of the ocean clustered next to hydrothermal vents. Living at temperatures up to 176°F (80°C), these animals are the most heat-tolerant known to science, and they aren't even unicellular. Because they live in such isolated areas, many extremophiles weren't discovered until the 1970s.

Perhaps the most memorable example of an extremophile is a bacteria, Streptococcus mitis, that was found embedded in the camera of the lunar probe Surveyor 3 by Apollo 12 astronauts. This bacteria had survived on the surface of the moon for three years. Scientists sometimes study extremophiles as potential models of what life might look like or how it would operate on other worlds.