Atmospheric　humic-like　substances　(HULIS)　are　not　only　an　unresolved　mixture　of　macroorganic　compounds　but　also　powerful　chelating　agents　in　atmospheric　particulate　matters　(PMs);　impacting　on　both　the　properties　of　aerosol　particles　and　health　effects　by　generating　reactive　oxygen　species　(ROS).　Currently,　the　interests　of　HULIS　are　intensively　shifting　to　the　investigations　of　HULIS-metal　synergic　effects　and　kinetics　modeling　studies,　as　well　as　the　development　of　HULIS　quantification,　findings　of　possible　HULIS　sources　and　generation　of　ROS　from　HULIS.　In　light　of　HULIS　studies,　we　comprehensively　review　the　current　knowledge　of　isolation　and　physicochemical　characterization　of　HULIS　fromatmospheric　samples　as　well　as　HULIS　properties　(hygroscopic,　surface　activity,　and　colloidal)　and　possible　sources　of　HULIS.　This　reviewmainly　highlights　the　generation　of　reactive　oxygen　species　(ROS)　fromPMs,　HULIS　and　transition　metals,　especially　iron.　This　review　also　summarized　the　mechanism　of　ironorganic　complexation　and　recent　findings　of　OH　formation　from　HULIS-metal　complexes.　This　review　will　be　helpful　to　carry　out　the　modeling　studies　that　concern　with　HULIS-transition　metals　and　for　further　studies　in　the　generation　of　ROS　from　HULIS-metal　complexes.　(C)　2017　The　Research　Center　for　Eco-Environmental　Sciences,　Chinese　Academy　of　Sciences.　Published　by　Elsevier　B.V.