The need to characterize imperfectly contacting interfaces is encountered in a wide variety of scientific and engineering problems, as illustrated in Fig. 1. Parts (a)–(c) illustrate the stages of diffusion bonding, [1,2] in which the condition of the interface evolves from one of isolated contacts at the initial stages of bonding through one containing distributed micropores, which exist during intermediate stages, to a state in which the material is fully bonded but in which there may be some near-interface microstructural variations. The ability to determine the degree to which bonding has passed through these conditions is presently needed in NDE of products after manufacturing and could be extended to process control if appropriate in-situ sensors were available. Part (d) illustrates the partial contact that can occur during fatigue crack growth [3]. Ideally, one might think of the surfaces of the fatigue crack as being free of stress. However, plastic deformation of ligaments during the failure process, motion of oxide debris, and shearing of the two faces of the crack can all lead isolated regions of contact along the crack face. These contacts are important in the fracture process since they can influence the loads which act on the tip of the crack during fatigue. From the NDE perspective, they can influence the strength of an ultrasonic signal which might be scattered by the crack. Part (e) illustrates a classical problem in tribology. Knowledge of the true area of contact between two surfaces is an essential ingredient in relating macroscopic variables such as applied force to the microscopic force and deformation distributions that exist at the interface [4]. Finally, as shown in part (f), interface design is an important aspect of the development of advanced engineering materials such as composites. It is often desirable to control the mechanical behavior of the interface is such a way that the overall response of the material is optimized, and the presence of pores, precipitates or other impurities may play an important role in controlling this interface behavior. Nondestructive characterization of such interfacial conditions is important in both ensuring the quality of material after fabrication and in assessing the degree to which service induced damage has occurred.