Ammonia nitrogen (NH ₃ /NH ₄ ⁺ ) polluted wastewater raises a serious threat to the safety of aquatic system. In addition to eutrophication, the presence of NH ₃ /NH ₄ ⁺ in the aquatic body reduces chlorine disinfectant efficiency during water treatment. Concerns regarding other conventional NH ₃ ‐N removal processes, to tackle these contaminants, advanced oxidation processes (AOPs) have been applied to remove ammonia nitrogen. The AOPs utilizes hydroxyl radical (•OH) for oxidation and have received considerable attention during the last few years in wastewater treatment, research technologies, and development. These processes are used to gradually reduce ammonia nitrogen to innocuous products with the help of high reactivity of hydroxyl radicals. In this chapter, a systematic study is carried out with the focus on the effects of ammonia nitrogen on the growth, physiology, biochemistry, and immune response of aquatic species. Moreover, experimentations and working procedures that can be used to remove ammonia nitrogen by AOP application are been discussed. This chapter also reviews recent findings and observations for the removal of ammonia nitrogen by photocatalysis and ozonation techniques and provide some recommendations for future research works.