Since the 1990s, the use of carbon fibre reinforced polymer (CFRP) materials has made large advances in the civil engineering construction field, particularly in the bridge upgrade and rehabilitation. To date, there are many applications in metallic beams strengthened with a bonded CFRP plate. However, the design method and technique are not sophisticated, and some specific areas require further research. In this technique, the structural adhesive bonding is the most distinct element to be considered. The main aspects discussed in this thesis are interfacial stress analysis, the behaviour under static loading and the fatigue performance. Firstly, an analytical solution and a numerical procedure are presented to calculate the stresses in beams reinforced by CFRP plates with a uniform thickness as well as with ends that are tapered. Finite element analysis was employed to validate the analytical results and a parametric study was carried. Secondly, ten retrofitted steel beams were tested under static loading to investigate the factors influencing the strength, such as different length and thickness of CFRP plates, the effects of the end taper and of the adhesive spew fillets beyond the plate ends. The test results were also used to validate the results of the analytical stress analysis. Thirdly, nine further retrofitted steel beams were tested under fatigue loading. Backface-strain technique was applied to monitor crack initiation and growth, and to assess the effect of the spew fillets. An S-N curve based on the peak interfacial stresses is proposed to predict the fatigue performance of the retrofitted beams. Finally, Finite element analysis was employed to investigate the geometric effect of the spew fillet and of the taper at the plate end of the retrofitted beam. The results confirm that the spew fillet size has a large effect on the strain in the plate ends. In the conclusion, a simple and integrated design method/technique for steel beams reinforced by bonded CFRP plates is proposed, based on the findings obtained from the studies presented.