The recent increase in use of composite construction has necessitated an improvement in the level of knowledge of how composite systems work. The purpose of the work for this thesis is to determine the behavior of frames employing semi-rigid composite connections and to develop a design methodology for these frames which is accurate and practical for use in the design office. This will require first an exact analytical solution to the deflection and distribution of forces within the frame. This solution must incorporate accurate representations of the moment-rotation curves for semi-rigid composite connections, which have been determined by experimental testing and finite element modelling. Frames relying on semi-rigid connections to provide their lateral support are more flexible than rigid frames so the exact solution must incorporate second-order analysis, taking into account the destabilizing effect of larger displacements. Once an exact solution is determined, simplifying assumptions and approximations must be made in order to make the analysis sufficiently simple to incorporate in design. The design approach will be checked against the exact solution and frames designed using it will be compared for economy and reliability against those designed using traditional methods.