The　overgrowth　of　the　glycogen　accumulating　organisms　(GAOs)　has　become　one　of　the　main　reasons　causing　the　unsteady　operation　of　enhanced　phosphorus　removal　(EBPR)　processes.　Therefore,　the　metabolic　mechanism　of　GAOs　as　well　as　their　microbial　morphology　and　community　is　introduced　for　better　understanding　the　exact　microbial　community　of　EBPR.　GAOs　can　use　synthetic　glycogen　as　the　energy　source　to　assimilate　volatile　fatty　acids　(VAFs)　anaerobically,　resulting　in　a　competition　with　phosphorus　accumulation　organisms　(PAOs)　for　VAFs.　Also,　they　will　become　the　dominate　population　under　un-identified　conditions　by　outcompeting　PAOs,　ultimately　leading　to　the　failure　of　EBPR.　The　studies　show　that　the　influent　organic　composition,　P/C,　pH,　temperature　and　nitrites　are　the　key　factors　impacting　the　competition　between　PAOs　and　GAOs.　Within　the　certain　ranges　of　the　above　operational　parameters,　the　accumulation　of　GAOs　occurs　and　the　PAOs　activities　will　be　completely　inhibited.