The discovery of ammonia-oxidizing archaea (AOA), now generally recognized to exert primary control over ammonia oxidation in terrestrial, marine, and geothermal habitats, necessitates a reassessment of the nitrogen cycle. In particular, the unusually high affinity of marine and terrestrial AOA for ammonia indicates that this group may determine the oxidation state of nitrogen available to associated micro- and macrobiota, altering our current understanding of trophic interactions. Initial comparative genomics and physiological studies have revealed a novel, and as yet unresolved, primarily copper-based pathway for ammonia oxidation and respiration distinct from that of known ammonia-oxidizing bacteria and possibly relevant to the production of atmospherically active nitrogen oxides. Comparative studies also provide compelling evidence that the lineage of Archaea with which the AOA affiliate is sufficiently divergent to justify the creation of a novel phylum, the Thaumarchaeota.