The preceding three chapters have dealt with the elastic behaviour of composites. Among the assumptions made in most of these treatments is that the interfacial bond is ‘perfect’. This means that there is no debonding, cracking or sliding – in fact, no elastic or inelastic processes of any description. In practice, many important phenomena may take place at the interface, depending on its structure and the stresses generated there. These processes tend to promote plastic deformation of the matrix and can also influence the onset and nature of failure. Before treating the strength and fracture behaviour of composites (Chapters 8 and 9), it is necessary to consider the interface and examine how its response can be characterised and influenced. In the present chapter, the meaning and measurement of bond strength are described. This is followed by an outline of the formation of interfacial bonds in various systems and a summary of the techniques used to influence the bonding characteristics.