How　to　prevent　and　control　filamentous　bulking　sludge　has　being　a　research　focus　and　attracted　much　attention.　To　date　despite　the　extensive　research　that　has　been　done　on　bulking　sludge,　filamentous　bulking　still　occurs　world-wide　and　a　comprehensive　solution　does　not　seem　to　be　available.　Particularly,　there　are　few　studies　about　making　use　of　the　characteristics　of　filamentous　bacteria　and　achieving　energy　saving　by　filamentous　bulking.　Limited　filamentous　bulking,　a　novel　method　for　energy　saving　while　equal　or　better　treatment　performance　by　allowing　slight　and　controlled　filamentous　bulking　sludge　at　low　dissolved　oxygen　(DO),　was　proposed　based　on　full-scale　field　observations.　The　practical　operation　showed　that　limited　filamentous　bulking　resulted　from　a　decline　in　DO　concentration.　COD,　SS　and　TP　removal　could　be　enhanced　and　energy　saving　could　be　achieved　by　limited　filamentous　bulking　at　low　DO.　The　derivation　of　limited　filamentous　bulking　is　introduced　firstly,　and　then　the　theoretical　fundamentals　of　the　technique　of　limited　filamentous　bulking　are　presented　and　analyzed,　including　the　occurrence　of　limited　filamentous　bulking　caused　by　low　DO,　the　enhanced　effect　of　filamentous　bacteria　on　pollutants　removal　and　the　energy　saving　mechanism　by　using　limited　filamentous　bulking.　Besides,　the　paper　provides　some　new　perspectives　about　the　application　and　research　direction　of　limited　filamentous　bulking　in　future.