Thaumarchaeota represent a unique phylum within the domain Archaea that embraces ammonia-oxidizing organisms from soil, marine waters, and hot springs as well as many lineages represented only by environmental sequences from virtually every habitat that has been screened. All cultivated Thaumarchaeota perform the first step in nitrification. They live under autotrophic conditions and fix CO2, but some are dependent on the presence of other bacteria or small amounts of organic material. Different from bacterial ammonia oxidizers, all cultivated Thaumarchaeota are adapted to comparably low amounts of substrate (ammonia) and inhabit not only moderate but also extreme environments, such as hot springs and acidic soils. All cultivated strains contain tetraether lipids with crenarchaeol, a Thaumarchaeota-specific core lipid.