Abstract: This chapter summarises recent research on the strengthening of thin-walled steel structures with fibre-reinforced polymers. The focus is on steel square hollow sections strengthened with carbon fibre-reinforced polymer. An extensive series of experiments with a wide range of thin-walled steel section geometries where the exterior surfaces of the tube walls were strengthened with bonded carbon fibres, is discussed. Experiments were performed under quasi-static axial compression to investigate strengthening with regards to elastic buckling and compression strength, and under axial impact to investigate strengthening with regards to dynamic axial crushing and associated crashworthiness indicators. Design models are developed and compared with the experimental results, and optimisation is discussed.