External bonding of fiber-reinforced polymer (FRP) laminates (plates or sheets) for strengthening or retrofitting concrete structures is a bond-critical application. In FRP bonding systems, the performance of the FRP–concrete or steel interface in providing an effective stress transfer is crucial. The attachment of FRP laminates to a concrete substrate can result in interfacial failure modes apart from the conventional flexural failure. In addition, these shifts in failure modes can alter the strength and ductility of the strengthened system. Therefore the fracture theory is introduced in this chapter to discuss the FRP–concrete interface properties. Several studies have presented various failure modes observed in retrofitted civil-engineering structures. Among these, the crack-induced debonding failure mode is the most common in FRP flexural or shear-strengthened concrete members. This is extremely important in the strengthening of concrete members with externally bonded FRP laminates. According to interfacial stress analysis, the interfacial problem may be idealized and studied as an FRP–concrete joint with FRP laminates bonded to the concrete surface and subjected to tension. In this chapter, the bond behavior of similar idealized FRP–concrete joints and design proposals for similar joints are discussed comprehensively.