In　recent　years,　microplastics　(MPs)　have　attracted　worldwide　attention　as　emerging　pollutants,　and　wastewater　treatment　plants　are　among　the　environment＇s　most　important　sources　of　MPs.　This　study　aimed　to　summarize　MPs　effects　on　various　aspects　of　sludge　systems.　The　results　showed　that　MPs　inhibited　substance　transformation　in　sludge　systems,　with　greater　inhibition　observed　at　higher　concentrations　and　smaller　MP　particle　sizes.　Moreover,　low　concentrations　of　MPs　promote　extracellular　polymeric　substance　(EPS)　secretion,　whereas　high　concentrations　suppress　EPS　secretion,　destroying　the　sludge　structure.　Granular　sludge　systems　exhibit　higher　resistance　to　MPs　than　activated　sludge　systems　owing　to　their　layered　structures.　Micrometer-sized　MPs　primarily　inhibit　the　dewatering　performance　of　activated　sludge　through　physical　crushing,　while　nano-sized　MPs　primarily　affect　sludge　dewatering　through　biological　effects.　However,　for　granular　sludge　system,　micrometer-sized　MPs　were　unable　to　penetrate　the　granular　sludge　interior,　their　affinity　and　aggregation　capabilities　enabled　them　to　accumulate　on　the　sludge　surface.　In　contrast,　nano-sized　MPs　can　enter　the　interior　of　granular　sludge　and　impair　mass-transfer　pathways,　ultimately　resulting　in　toxic　effects.　Furthermore,　MPs　induce　various　toxic　effects　in　sludge　systems　(single　and　combined　toxic　effects).　These　toxic　effects　affect　the　expression　of　key　enzymes　and　functional　genes,　leading　to　changes　in　microbial　communities.　Because　of　the　layered　structure　of　granular　sludge　systems,　MPs　may　initially　affect　the　microbial　community　structure　outside　the　granules,　with　less　impact　on　the　internal　microorganisms.　However,　the　specific　mechanism　still　needs　to　be　explored.　Finally,　this　study　presents　research　questions　and　directions　that　require　further　investigation.