Chemoautotrophs are organisms that obtain their energy from a chemical reaction (chemotrophs) but their source of carbon is the most oxidized form of carbon, carbon dioxide (CO2). The best known chemoautotrophs are the chemolithoautotrophs that use inorganic energy sources, such as ferrous iron, hydrogen, hydrogen sulfide, elemental sulfur or ammonia, and CO2 as their carbon source. All known chemoautotrophs are prokaryotes, belonging to the Archaea or Bacteria domains. They have been isolated in different extreme habitats, associated to deep-sea vents, the deep biosphere, or acidic environments. This form of energy conservation is considered one of the oldest on Earth. These microorganisms are of astrobiological interest because they could develop in the extreme conditions existing in different extraterrestrial planetary bodies, like Mars or Europa.