The present work is dedicated for concrete-fillcd stecl tubes. the main parameters studied arc : the slcnderness, the cross section shapo, the stecl residual stresses, the loading conditions and the strength of concrete and steel. chapter one gives a review of works donc in different contrics. chapter two is dedicated for the bending moment-thrust-curvature generation, which is necessary for the study of the equilibrium of composite columns. this includes the cffccts of the stecl residual stresses. in chapter three, the carrying capacity of such composite columns under axial load is investigated using the tangent modulus method. the steel mean residual stress is introduced, but no consideration was given to the lack of straightness. the tangent modulus method gave good predictions for axially loaded columns. as this method does not include the gcomctrical imperfections, in chapter four, a closed form solution is suggested based on a sine wave deflected shape. the equilibrium of the column is satisfied only at mid-span by an iterative process. à variable path of residual stresses is taken and the lack of straightness ot ec column is included. the predicted loads using this method were higher than those obtained experimentally. it is believed that such method is not appropriate for such columns. a non-linear finite element program was suggested by the author in chapter five, which was validated with experimental results. good results were obtained by this program in predicting the failure loads and the response of such composite columns under different boundary conditions. discussions are presented in chapter six from which it was concluded that the finite element model is an acceptable model for intermediate and long columns. more can be done for short columns by using a three dimensionnal model. the experimental results are given in chapter seven for circular long composite columns and cold formed short rectangular concrete-filled columns. both theoritical and experimental results were in good agreement with the ec4 predictions for axially and eccentnically loaded columns with single curvature. however, the ec4 loads were higher in the case of columns with double curvature bending. more experimental results are needed for double curvature bending.