Innovation　in　decarbonizing　wastewater　treatment　is　urgent　in　response　to　global　climate　change.　The　practical　implementation　of　anaerobic　ammonium　oxidation　(anammox)　treating　domestic　wastewater　is　the　key　to　reconciling　carbon-neutral　management　of　wastewater　treatment　with　sustainable　development.　Nitrite　availability　is　the　prerequisite　of　the　anammox　reaction,　but　how　to　achieve　robust　nitrite　supply　and　accumulation　for　mainstream　systems　remains　elusive.　This　work　presents　a　state-of-the-art　review　on　the　recent　advances　in　nitrite　supply　for　mainstream　anammox,　paying　special　attention　to　available　pathways　(forward-going　(from　ammonium　to　nitrite)　and　backward-going　(from　nitrate　to　nitrite)),　key　controlling　strategies,　and　physiological　and　ecological　characteristics　of　functional　microorganisms　involved　in　nitrite　supply.　First,　we　comprehensively　assessed　the　mainstream　nitrite-oxidizing　bacteria　control　methods,　outlining　that　these　technologies　are　transitioning　to　technologies　possessing　multiple　selective　pressures　(such　as　intermittent　aeration　and　membrane-aerated　biological　reactor),　integrating　side　stream　treatment　(such　as　free　ammonia/free　nitrous　acid　suppression　in　recirculated　sludge　treatment),　and　maintaining　high　activity　of　ammonia-oxidizing　bacteria　and　anammox　bacteria　for　competing　oxygen　and　nitrite　with　nitrite-oxidizing　bacteria.　We　then　highlight　emerging　strategies　of　nitrite　supply,　including　the　nitrite　production　driven　by　novel　ammonia-oxidizing　microbes　(ammonia-oxidizing　archaea　and　complete　ammonia　oxidation　bacteria)　and　nitrate　reduction　pathways　(partial　denitrification　and　nitrate-dependent　anaerobic　methane　oxidation).　The　resources　requirement　of　different　mainstream　nitrite　supply　pathways　is　analyzed,　and　a　hybrid　nitrite　supply　pathway　by　combining　partial　nitrification　and　nitrate　reduction　is　encouraged.　Moreover,　data-driven　modeling　of　a　mainstream　nitrite　supply　process　as　well　as　proactive　microbiome　management　is　proposed　in　the　hope　of　achieving　mainstream　nitrite　supply　in　practical　application.　Finally,　the　existing　challenges　and　further　perspectives　are　highlighted,　i.e.,　investigation　of　nitrite-supplying　bacteria,　the　scaling-up　of　hybrid　nitrite　supply　technologies　from　laboratory　to　practical　implementation　under　real　conditions,　and　the　data-driven　management　for　the　stable　performance　of　mainstream　nitrite　supply.　The　fundamental　insights　in　this　review　aim　to　inspire　and　advance　our　understanding　about　how　to　provide　nitrite　robustly　for　mainstream　anammox　and　shed　light　on　important　obstacles　warranting　further　settlement.