There are three main classes of defect which occur in adhesive joints: complete disbonds, voids or porosity in the adhesive layer, poor cohesion (ie a weak adhesive layer) and poor adhesion (ie a weak interface between the adhesive layer and one or both adherends). The detection of disbonds, voids and porosity generally presents few problems and significant progress has been made towards the development of techniques for monitoring the cohesive properties of the adhesive layer [1]. However, there is no satisfactory method for the detection of a weak interface between the adhesive and the adherend(s) and this remains one of the major challenges in NDE. It is the interlayer which is affected by the common problem of slight contamination due to, for example, grease on the adherend surfaces prior to bonding. The adhesive/adherend interface is particularly important in aluminium-aluminium joints in which an inappropriate interface structure can cause greatly enhanced susceptibility to environmental attack [2]. Inspection of the interlayer is difficult because it is frequently only of the order of 1µm thick, compared with an adhesive layer thickness of the order of 100 µm.