The　thermocatalytic　decomposition　(TCD)　of　methane　is　considered　as　a　milestone　towards　the　production　of　valuable　COx-free　hydrogen　and　carbon　nanomaterials　without　the　use　of　steam　or　O-2.　Previous　reviews　have　been　aimed　at　methane　decomposition　over　the　different　catalysts,　such　as　nickel-based　catalysts,　non-nickel-based　catalysts,　metal　oxide-supported　catalysts,　and　carbon-supported　catalysts.　The　Ni-based　catalysts　are　suitably　applied　for　methane　TCD　process　due　to　their　high　activity　and　low　cost.　However,　the　loss　of　activity　and/or　stability　with　reaction　time　is　one　of　the　most　notable　challenges　in　the　use　of　Ni-based　catalysts,　and　a　number　of　studies　on　the　roles　of　various　factors　in　overcoming　such　a　problem　can　be　found　in　the　literature.　Recently,　the　use　of　the　second　metal　as　a　promoter　to　control　catalyst　deactivation　has　attracted　much　attention.　The　present　review　focuses　on　classification　of　the　different　promoters　based　on　the　periodic　table　of　elements,　such　as　alkali　metals,　alkaline　earth,　transition　metals,　noble　metals,　and　rare　earth　metals,　and　makes　a　detailed　discussion　on　promotional　roles　in　influencing　their　physicochemical　properties　and　catalytic　performance　of　the　Ni-based　catalysts.　The　generalized　structure-performance　relationship　of　the　metals-doped　catalysts　may　give　an　appreciated　reference　to　the　design　of　catalysts　with　highly　pure　hydrogen　production　and　carbon　nanomaterials.　In　addition,　this　review　also　covers　the　works　on　effects　of　the　promoters　on　nature　and　morphology　of　the　formed　carbon　nanomaterials.　The　use　of　transition　metals　(Fe,　Co　or　Cu),　noble　metal　(Pd　or　Pt),　and　rare　earth　metal　(La)　with　a　suitable　loading　as　a　promoter　influenced　performance　and　lifespan　of　the　catalyst　and　the　interaction　of　Ni　particles　with　the　support.　Among　these　promoters,　Cu,　Pd,　La,　and　Cu-Pd　as　a　dopant　have　demonstrated　superior　performance,　which　was　attributed　to　the　capability　of　these　elements　in　prohibiting　carbon　accumulation　on　the　active　Ni　components.　(C)　2021　Hydrogen　Energy　Publications　LLC.　Published　by　Elsevier　Ltd.　All　rights　reserved.