Overall, species of archaea inhabit a wider range of environments than either bacteria or eukaryotes, from extreme heat to extreme cold, from high pH to extreme acid. Their metabolism includes unique capacities, such as methanogenesis. Yet we probably know less about the actual scope of Archaea than we do about the other two domains because so many members remain uncultured. We recognize two major divisions: the crenarchaeotes, including sulfur- metabolizing hyperthermophiles as well as marine mesophiles and psychrophiles; and the euryarchaeotes, including methanogens, halophiles, and sulfur- metabolizing extremophiles. New deeply branching isolates continue to be discovered, potentially representing entire new divisions. Much of what we call Archaea remains to be explored.