Reinforced concrete (RC) beams strengthened in flexure using externally bonded (EB) or near-surface mounted (NSM) fiber-Reinforced Polymers (FRP) and metals have gained considerable interest over the last few decades. As few of the previous review papers concerned with NSM elements or both EB and NSM methods so this paper aimed to review the previous research that handled the behavior of RC beams strengthened by NSM and EB techniques, compared their advantages and disadvantages and reported the future recommendations. Moreover, the review focused on the effect of strengthening materials (metallic and FRP materials), strengthening method, strengthening elements characteristics (shape and area), and any other factors affecting the performance of the strengthened RC beams in each technique. The review demonstrated that the strengthening element’s failure mode significantly affects the RC beams’ load-carrying capacity strengthened by EB or NSM. Moreover, the type of FRP materials and the end anchorage significantly affect the load efficiency of the strengthened RC beams depending on the type of failure mode.