Application of carbon fiber-reinforced polymer (CFRP) composites for retrofitting reinforced concrete structures has been extensively investigated and used in practice. Many studies demonstrated the beneficial influence of such composite materials for the flexural and shear strengthening of concrete girders, as well as for confinement of concrete columns. However, the strengthening techniques and the accompanying theories for metallic structures have not been developed as thoroughly as those for concrete structures. There are several differences between the behavior of bonded joints in CFRP-strengthened concrete and metallic members, which will be briefly explained in this paper. Furthermore, one of the main aims of the paper is to give an overview on different techniques for carbon-fibre reinforced polymer (CFRP) strengthening of steel plates and beams. Different bonded and un-bonded retrofit systems will be discussed with particular focus on application of pre-stressed un-bonded retrofit (PUR) systems. Furthermore, some details about design and testing procedure of a new so called flat PUR (FPUR) system will be given. Finally, the paper gives some details about CFRP strengthening and wireless sensor monitoring of two old metallic bridges in Switzerland and Australia.