Literature was reviewed on oxidation mechanism of ammonia nitrogen in oxygen concentration limited and anoxic conditions. It was observed that chemolithotrophic bacteria of Nitrosomonas genus oxidising ammonia nitrogen may use ammonium as a donor of electrons and simultaneously oxygen and nitrite as electron acceptors. The products of ammonia nitrogen oxidation, except nitrates, are hydroxylamine, nitrogen oxides and molecular nitrogen. Mechanisms of the process are presented. The process of heterotrophic nitrification is characterised. Factors stimulating the process of simultaneous nitrification and denitrification in activated sludge as well as the stoichiometry and reaction kinetics of the process were analysed. Finally, the currently emerging research trends were presented.