The　detection　of　gas　molecules　is　critical　for　environmental　monitoring,　chemical　process　control,　agriculture,　and　medical　applications.　Therefore,　gas　sensors　and　electronic　noses　(e-nose)　are　widely　studied　by　researchers　all　over　the　world.　Graphene　has　been　considered　to　be　a　promising　gas　detection　material　due　to　its　special　electronic　properties,　which　are　strongly　influenced　by　the　adsorption　of　extrinsic　molecules.　Doping　of　metal　oxides　and　nanometal　particles　has　also　been　extensively　studied　and　their　electrical　property　is　highly　sensitive　to　the　properties　of　absorbed　gases.　Carbon　nanotubes　(CNTs)　are　expensive　but　have　advantages　of　high　sensitivity,　good　reversibility,　and　nice　stability.　Several　research　groups　have　studied　the　mixed　structure　of　the　above　three　materials　with　their　derivatives　blended,　which　show　improved　gas　sensing　capabilities.　This　review　summarizes　the　state-of-the-art　progresses　in　the　research　on　gas　sensors　and　e-nose,　based　on　graphene,　metal　oxide,　and　CNTs.