Biological　nitrogen　removal　is　widely　applied　to　reduce　the　discharge　of　inorganic　nitrogen　and　mitigate　the　eutrophication　of　receiving　water.　However,　nitrogen　loss　is　frequently　observed　in　wastewater　treatment　systems,　yet　the　underlying　principle　and　potential　enlightenment　is　still　lacking　a　comprehensive　discussion.　With　the　development　and　application　of　novel　biological　technologies,　there　are　increasing　achievement　in　the　deep　understanding　and　mechanisms　of　nitrogen　loss　processes.　This　article　reviews　the　potential　and　novel　pathways　of　nitrogen　loss,　occurrence　mechanisms,　influential　factors,　and　control　strategies.　A　survey　of　recent　literature　showed　that　3%∼73%　of　nitrogen　loss　beyond　the　nitrogen　budget　can　be　ascribed　to　the　unintentional　presence　of　simultaneous　nitrification/denitrification,　partial　nitrification/anammox,　and　endogenous　denitrification　processes,　under　low　dissolved　oxygen　(DO)　and　limited　available　organic　carbon　source　at　aerobic　conditions.　Key　influential　parameters,　including　DO,　aeration　strategies,　solid　retention　time　(SRT),　hydraulic　retention　time　(HRT),　temperature　and　pH,　significantly　affect　both　the　potential　pathways　of　nitrogen　loss　and　its　quantitative　contribution.　Notably,　the　widespread　and　spontaneous　growth　of　anammox　bacteria　is　an　important　reason　for　ammonia　escape　at　anaerobic/anoxic　conditions,　leading　to　7%∼78%　of　nitrogen　loss　through　anammox　pathway.　Moreover,　the　unwanted　nitrous　oxide　(N2O)　emission　should　also　be　considered　as　a　key　pathway　in　nitrogen　loss.　Future　development　of　new　nitrogen　removal　technologies　is　proposed　to　suppress　the　generation　of　harmful　nitrogen　losses　and　reduce　the　carbon　footprint　of　wastewater　treatment　by　controlling　key　influential　parameters.　Transforming　＇unintentional　observation＇　to　＇intentional　action＇　as　high-efficiency　and　energy-efficient　nitrogen　removal　process　provides　a　new　approach　for　the　development　of　wastewater　treatment.　©　2023　Elsevier　Ltd