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This thesis concerns a study including testing, analysis and design on beam-to column connections for the frames of pultruded glass fibre reinforced plastic profiles. The research consists of the two principal aspects of a laboratory test programme to determine connection behaviour and the formulation of a plane frame analytical tool to determine the effect of real connection on frame behaviour. The laboratory tests involved three different 10 inch beam-to-column connections which can be categorised as having a moment-rotation behaviour that is pinned. These connections had web cleats and the method of connection was by bolting, or bolting combined with adhesive bonding. The test configuration had a central column and two beams in a cruciform arrangement. Loading was applied at the ends of the cantilever beams in such a way that the two identical connections experienced the same moment and shear force. The non-linear moment-rotation characteristics for the connections were determined and the modes of failure established. Another four tests were conducted on different 8 inch beam-to-column connections which can be categorised as being semi-rigid. Two of these connections used steel cleats for the top and bottom seat cleats while the other two had these pieces manufactured of pre-preg glass FRP using a pressure moulding process. The details and the results of these connection tests are presented. The results of the three tests on pinned connections confirmed conclusions previously reported on similar tests where the beams and column were of the 8 inch wide flange profile. The current practice of recommending that the cleated webs have combined bonding and bolting is shown to provide little additional benefit over that when the connection is bolted. It is found that a semi-rigid connection with an acceptable moment-rotation behaviour can be obtained using steel angles for the connection pieces. One of the two 'all' FRP connections was also found to have an acceptable moment-rotation characteristic but would be too complicated for real applications. The search therefore continues for the all FRP connection details giving suitable connection behaviour. Combining the results from these experiments and the analysis of the failure modes, a number of futuristic connection designs are proposed. They include a thin shell cleat piece with curvature to increase its stiffness, six connection details using pieces that connect together by interlock and bonding (this approach reduces the need for bolting) and a radically novel system which does not mimic current steelwork practice. A new analytical method is presented which predicts the static response of nonlinear elastic plane frames of polymeric composite members. Options allow for horizontal and vertical loading, second-order deflections and connections with nonlinear moment-rotation characteristics. The matrix stiffness method uses a new approach to cope with the real connection behaviour. The other novel aspects of the analysis are shear deformable members and new stability functions which account for the shear deformation when determining second-order deflections. A number of new (< p) functions are derived to group the new stability functions, and these can readily be employed in the computing analysis. The analysis is successfully benchmarked against known semi-rigid frame problems of steelwork and with the limited results from a single experiment on a pultruded single bay frame. A serviceability beam line is presented for beam design and a moment equalized connection stiffness is obtained. Two unbraced and one braced frame problems are analysed in a parametric study to determine the effect on frame response of having connections with various properties. Live loading is vertical with a horizontal component. Results of overall sway deflection showed how the frames respond when the connection properties are changed from pinned, through semi-rigid, to fully fixed. The sensitivity of the connection stiffness on the overall frame behaviour is demonstrated by the sway and the midspan deflections results, and by the moment distribution in the members. The analysis is shown to provide useful information in our quest for the optimised connection design and their performance when viewed as an integral part of a whole frame. To establish which connection details are best suited to pultruded frame construction recommendations for further research and development work are given.