Fiber-reinforced composite (FRP) materials are used to rehabilitate and retrofit steel-reinforced concrete (RC) members such as beams, columns, and so on. This technique is applied also for masonry, steel, and timber structures. A review of the behavior of RC constituent materials, i.e., concrete and steel, is reported first. The reader is assumed to be familiar with composite materials, which are described in detail in previous chapters. The FRP strengthening methodology presented here is consistent with American Concrete Institute (ACI) guidelines [288]. The main hypotheses and assumptions for the design of RC members are reported in accordance to ACI guidelines [289].