A variety of conventional NDI techniques, such as visual inspection, radiography, eddy current and ultrasonics, have already been developed and implemented to detect fatigue cracks in aircraft structures. There is a continuing requirement to decrease the minimum detectable crack length so as to assure detection prior to crack criticality and component failure. It is often necessary to detect small fatigue cracks under the fastener head without prior removal of the fastener from the structure [1–3]. The location of the crack initiation site is dependent upon the local stress distribution in the vicinity of the fastener and the surface condition of the hole and the countersink, but may generally be considered to occur at one of three possible sites: at the countersink, at the faying surface or in the bore of the fastener hole. Despite a significant, long term level of activity, a fast, reliable means of detecting partially or totally obscured fatigue cracks at these sites without the removal of the fastener does not exist.