Anammox, anaerobic ammonium oxidation with nitrite, is now recognized as an important process in the marine nitrogen cycle. The bacteria conducting anammox are highly specialized and appear to belong to the Planctomycetales. The process has now been found in a range of environments including marine sediments, sea ice and anoxic water columns, and it may be responsible for up to 50% of the global removal of fixed nitrogen from the oceans.