Energy　crisis　and　environmental　problems　urgently　drive　the　proposal　of　new　strategies　to　improve　human　wellbeing　and　assist　sustainable　development.　To　this　end,　scientists　have　explored　many　metal　oxides-based　photocatalysts　with　high　stability,　low　cost,　earth　abundance,　and　potentially　high　catalytic　activity　relevant　for　key　applications　such　as　H2O　splitting,　CO2　reduction,　N2　fixation,　and　advanced　oxidation　of　pollutants.　In　these　metal　oxides,　oxygen　vacancies　(OVs)　are　ubiquitous　and　intrinsic　defects　with　pronounced　impacts　on　the　physicochemical　properties　of　the　catalysts,　which　may　open　new　opportunities　for　obtaining　efficient　metal　oxides.　The　thorough　understanding　of　the　structural　and　electronic　nature　of　OVs　is　necessary　to　determine　how　they　serve　as　catalytically　active　sites.　In　this　review,　we　summarize　the　origin　of　OVs,　the　strategies　to　introduce　OVs,　as　well　as　the　fundamental　structure-activity　relationships　to　relate　these　crystal　defects　to　catalyst　properties　including　light　absorption,　charge　separation,　etc.　We　emphasize　the　mechanism　of　OVs　formation　and　their　effects　on　the　intrinsic　catalytic　characteristics　of　the　metal　oxides.　We　also　present　some　multicomponent　catalytic　platforms　where　OVs　contribute　to　catalysis　via　synergy.　Finally,　opportunities　and　challenges　on　engineering　defects　in　photocatalysts　are　summarized　to　highlight　the　future　directions　of　this　research　field.　©　2020,　Science　China　Press　and　Springer-Verlag　GmbH　Germany,　part　of　Springer　Nature.