Primitive single-celled organisms thought by some to be the oldest life-forms on Earth. They were first known for their ability to survive in a range of extreme environments, such as salty water, hot springs, and sulfur vents at the bottom of the ocean floor, though further research also finds Archaea in a variety of non-extreme environments.

There is some disagreement as to how the Archaea should be classified. They were originally categorized as a sub-group of Bacteria, but they have very few similarities with them. Some taxonomies place them in a domain of their own, based on the differences between their genetic code and those of the Eukaryotes and Prokaryotes. Others place them within the Monera. Thus, the main divisions within the Archaea (Crenarchaea, Euryarchaeota, and sometimes others such as Korarchaeota and Nanoarchaeota) are variously described as kingdoms, phyla, or even classes.

Single-celled organisms which are bounded by ether-linked lipid membranes which contain isoprinoid side chains instead of fatty acids. The most famous of the Crenarchaeota are the hyperthermophiles, organisms which thrive at temperatures close to the boiling point of water. Members of the crenarchaeal order Sulfolobales also live in extremely acidic waters. However, further research has shown that these organisms are also common in moderate-temperature water and soil.
Small single-celled organisms of a variety of shapes, including the methanogens and halophiles.

Most researchers group these organisms together based on molecular similarities of their RNA, though there are a variety of competing schemes.