Twenty years since the discovery of the Feammox process —ammonium oxidation under iron reduction— which has been observed in diverse environments, including agricultural soils, wetland, riverine and estuarine sediments, its role in- and contribution to the global nitrogen cycling remain elusive. The oxidation of ammonium to nitrite, favored under acidic conditions, and to nitrate, which is energetically more favorable have been reported, suggesting possibly distinct underlying mechanisms. The acidophilic Acidimicrobium sp. strain A6 (A6), linked to ammonium oxidation to nitrite, has been identified in many Feammox-active environments, and it is likely that other organisms capable of performing this reaction are yet to be identified. Conveners invite contributions that advance the understanding of the Feammox process in the global nitrogen cycle, new mechanistic insights, interactions with other iron reducing or nitrogen bacteria, alternate electron acceptors, environmental parameters that favor the Feammox process, or novel applications in biotechnology and water quality management.