This　paper　reports　an　experimental　and　numerical　study　of　the　cross-sectional　behaviour　of　cold-formed　high　strength　steel　(HSS)　square　and　rectangular　hollow　section　(SHS　and　RHS)　members.　Six　stub　column　tests,　six　3-point　and　three　4-point　bending　tests　have　been　carried　out,　followed　by　the　development　of　accurate　finite　element　(FE)　models　and　a　parametric　study.　Based　on　the　test　and　FE　results,　the　design　rules　for　slender　cross-sections　in　European,　American,　Australian　and　Chinese　standards　and　the　GSRM　method　have　been　assessed　via　reliability　analyses.　A　new　equation　for　deriving　the　cross-sectional　slenderness　of　SHS　and　RHS　considering　the　influence　of　corner　radius　and　new　area　reduction　factors　for　designing　slender　cold-formed　S900　(and　above)　SHS　and　RHS　have　also　been　proposed.