Tailoring　reactive　oxygen　species　(ROS)　in　advanced　oxidation　processes　(AOPs)　in　a　controlled　manner　is　essential　in　chemical　synthesis　and　environmental　applications.　In　this　paper,　we　begin　with　an　overview　of　four　ROS,　and　summarize　methods　and　various　precursors　(e.g.,　O2,　H2O2,　and　persulfate)　to　generate　particular　ROS.　We　then　examine　the　use　of　the　ROS　in　the　degradation　of　pollutants　and　in　the　synthesis　of　value-added　chemicals.　We　highlight　the　use　and　mechanism　response　of　transition　metal　catalysts,　generally　developed　with　defect　engineering,　in　advancing　AOPs.　We　conclude　with　an　outlook　of　current　challenges　and　future　perspectives　of　applying　ROS　in　AOPs.　We　anticipate　this　review　to　inspire　researchers　to　develop　green,　safe,　and　efficient　ROS　systems　for　sustainable　environmental　applications.　©　2022　The　Royal　Society　of　Chemistry.