Fiber-reinforced polymer (FRP) materials have been emerged as very effective and reliable materials for the repair, strengthening, and retrofitting of existing reinforced concrete (RC) structures. After a brief introduction on the variety of FRP materials and strengthening techniques for applications on civil structures, the basic principles for designing different strengthening configurations with these materials are herein presented and discussed. First, the bond mechanism, at the base of the structural performance of each type of external strengthening with FRP materials, is discussed both for externally bonded reinforcements (EBR) usually applied in the shape of sheets and plates and for near surface-mounted (NSM) reinforcements usually applied in the shape of bars or strips. Then, flexural, shear, and confinement applications are examined in detail and the basic mechanical principles of performing safety verifications are illustrated with reference to both provisions from current international design guidelines and innovative theories from up to date scientific literature. Finally, an overview on the potentialities of FRP materials for seismic strengthening is presented.