Polymeric　carbon　nitrides　(PCNs)　are　a　species　of　metal-free　semiconductors　that　consists　of　only　Earth-abundant　carbon　and　nitrogen　elements.　The　semiconducting　properties　of　PCN　were　first　realized　as　a　stable　photocatalyst　for　hydrogen　and　oxygen　evolutions　by　splitting　water　under　light　illumination.　The　recent　investigations　have　shown　unique　characteristics　that　diverse　semiconducting　properties　of　PCN　can　be　achieved　through　conventional　co-polymerization　of　condensing　organic　co-monomers　with　typical　carbon-　and　nitrogen-abundant　precursors.　The　properties　of　the　organic　co-monomer　would　affect　the　photochemistry　of　the　as-modified　PCN.　The　diverse　semiconducting　manners　of　PCN　will　eventually　inspire　a　series　of　advanced　developments　for　materials　science,　not　only　in　photocatalysis,　but　also　in　biology,　electronics,　and　many　more　fields.　Copyright　©　2020　American　Chemical　Society.