The　escalating　production　of　waste　activated　sludge　(WAS)　presents　significant　challenges　to　wastewater　treatment　plants　(WWTPs).　Free　nitrous　acid　(FNA),　known　for　its　biocidal　effect,　has　gained　a　growing　focus　on　sludge　dewatering,　sludge　reduction,　and　resource　recovery　from　WAS　due　to　its　eco-friendly　and　cost-effective　properties.　Nevertheless,　there　have　been　no　attempts　made　to　systematically　summarize　or　critically　analyze　the　application　of　FNA　in　enhancing　treatment　and　resource　utilization　of　sludge.　In　this　paper,　we　provided　an　overview　of　the　current　understanding　regarding　the　application　potential　and　influencing　factors　of　FNA　in　sludge　treatment,　with　a　specific　focus　on　enhancing　sludge　dewatering　efficiency　and　reducing　volume.　To　foster　resource　development　from　sludge,　various　techniques　based　on　FNA　have　recently　been　proposed,　which　were　comprehensively　reviewed　with　the　corresponding　mechanisms　meticulously　discussed.　The　results　showed　that　the　chemical　oxidation　and　interaction　with　microorganisms　of　FNA　played　the　core　role　in　improving　resource　utilization.　Furthermore,　current　challenges　and　future　prospects　of　the　FNA-based　applications　were　outlined.　It　is　expected　that　this　review　can　refine　the　theoretical　framework　of　FNA-based　processes,　providing　a　theoretical　foundation　and　technical　guidance　for　the　large-scale　demonstration　of　FNA.　©　2024　Elsevier　Ltd