This paper deals with seismic design and ductility issues in concrete-filled steel tubular structures, such as columns supporting highway bridge superstructures. The basic characteristics of the thin-walled steel tubular structures are noted and the importance of concrete fill in improving strength and ductility capacity of such structures is explained. A seismic design method for ultimate strength and ductility evaluation of concrete-filled, thin-walled, steel tubular beam-columns is presented. The application of the method is demonstrated by comparing the computed strength and ductility of some cantilever beam-columns with test results. The method is applicable for both the design of new, and retrofitting of existing, thin-walled steel tubular beam-columns. The effects of some important parameters, such as width-to-thickness ratio, column slenderness ratio, height of infill concrete and residual stress, on the ultimate strength and ductility of thin-walled steel tubular beam-columns, are presented and discussed.