On the basis of culture studies, Archaea were thought to be synonymous with extreme environments. However, the large numbers of environmental rRNA gene sequences currently flooding into databases such as GenBank show that these organisms are present in almost all environments examined to date. Large sequence databases and new fast phylogenetic software allow more precise determination of the archaeal phylogenetic tree, but also indicate that our knowledge of archaeal diversity is incomplete. Although it is apparent that Archaea can be found in all environments, the chemistry of their ecological context is mostly unknown.