A　mechanics　model　is　developed　in　this　paper　for　concrete-filled　steel　CHS　(circular　hollow　section)　beam-columns.　A　unified　theory　is　described　where　a　confinement　factor　(ξ)　is　introduced　to　describe　the　composite　action　between　the　steel　tube　and　the　filled　concrete.　The　predicted　load　versus　deformation　relationship　is　in　good　agreement　with　test　results.　The　theoretical　model　was　used　to　investigate　the　influence　of　important　parameters　that　determine　the　ultimate　strength　of　concrete-filled　steel　CHS　beam-columns.　The　parametric　and　experimental　studies　provide　information　for　the　development　of　formulas　for　the　calculation　of　the　ultimate　strength　of　the　composite　beam-columns.　Comparisons　are　made　with　predicted　beam-columns　strengths　using　the　existing　codes,　such　as　LRFD-AISC-1999,　ALT-1997,　BS5400-1979　and　EC4-1994.