A large percentage of short - and medium-span steel bridges are deteriorating because of age and environmental effects. Although these bridges are still in service, many need strengthening because of increases in legal live load, loss of section, or both. The results of two investigations are presented - a laboratory study and a field study - in which carbon fiber-reinforced polymer (CFRP) plates were used to strengthen composite steel stringers. In the laboratory investigation, small-scale steel-concrete composite beams were tested; there were control beams (no damage or CFRP applied), damaged beams (a percentage of the bottom flange removed), and damaged beams with CFRP applied to the bottom flanges, webs, or both. In all cases the strength of all damaged and repaired beams was fully restored to the original undamaged state. Details on both the strengthening system and the behavior of undamaged, damaged, and repaired scale-model specimens are presented. On the basis of the laboratory results, a second project was undertaken in which an existing steel girder bridge was strengthened with CFRP plates. This bridge is a 150- × 30-ft three-span continuous rolled I-beam bridge in southwestern Iowa. The original noncomposite four-beam bridge was widened in 1965 by adding two composite beams. A recent rating of this bridge determined that several of the original beams were understrength in the positive moment regions; thus CFRP was bonded to the positive moment regions of the bottom flanges of the two original interior beams and the "new" exterior beams. At some locations on the exterior beams, the plates were installed on the top surface of the bottom flange to investigate the performance and in-service durability under detrimental environmental conditions. This bridge has been load tested three times - before and alter installation of the C FRP plates and approx-imately 1 year later - to determine the effectiveness of the strengthening system. Results are presented to illustrate this effectiveness.