Melanoidins,　as　macromolecular　heterogeneous　organic　polymers,　are　produced　from　the　Maillard　reaction　between　amino　and　carbonyl　groups　during　the　thermal　hydrolysis　pretreatment　(THP)　of　sludge.　The　brown　color　and　recalcitrance　of　melanoidins　pose　a　serious　threat　to　wastewater　treatment　systems,　such　as　invalidating　UV　disinfection　and　decreasing　the　efficiency　of　anaerobic　digestion;　thus,　they　have　gradually　received　much　concern　in　recent　years.　However,　currently　the　study　on　THP-origin　melanoidins　is　limited　by　a　lack　of　reliable　extraction　and　quantification　methods.　This　paper　presents　a　comprehensive　review　of　the　physical,　chemical,　and　biological　properties　of　melanoidins　from　different　sources　to　fill　the　research　gap　on　THP-origin　melanoidins.　The　adverse　effects　of　melanoidins　on　the　management　of　wastewater　and　sludge　are　discussed,　and　for　the　first　time,　special　attention　is　paid　to　the　potential　environmental　hazards　of　THP-origin　melanoidins　to　natural　ecosystems.　The　removal　technologies　of　melanoidins　are　summarized　and　compared　as　well.　Finally,　the　suggested　areas　that　future　studies　should　focus　on　are　provided.　This　review　is　dedicated　to　providing　guidance　on　melanoidin　research　and　management　for　the　better　development　of　the　THP　industry.