This chapter contains sections titled: Introduction Archaea everywhere? Abundant Archaea in the marine plankton Cenarchaeum symbiosum: a model marine crenarchaeote Methanotrophic Archaea in marine sediments Ubiquitous Crenarchaea in soils might be ammonia oxidizers Rice Cluster I Archaea are an important group of methanogens Archaea in freshwater Archaea in association with eukaryotes Archaeal genomes in large metagenomic datasets Conclusions and outlook