An apparatus and method for the nondestructive inspection of dielectric materials are disclosed. Monochromatic, phase coherent electromagnetic radiation, preferably in the 5-50 gigahertz frequency range (i.e., microwaves) impinges on the sample. In accordance with Snell's law, the microwaves are partly transmitted and partly reflected at each interface where the dielectric constant changes (e.g., where there are delaminations, cracks, holes, impurities, or other defects.) A portion of the reflected beam is combined with the signal reflected by the specimen being inspected. These two signals have the same frequency, but may differ in amplitude and phase. The signals combine to produce an interference pattern, a pattern that changes as the specimen changes, or as the position of the specimen changes relative to that of the detector. Appropriate processing of the interference signal can greatly improve the signal-to-noise ratio. The detector may be scanned relative to the specimen at any desired speed, and the scanning speed need not be uniform. The detection technique is based on interference between reflected and reference microwaves having substantially the same frequency. This technique can detect cracks, voids, foreign material inclusions (e.g., water or oil), thickness changes, delaminations, changes in dielectric constant (which in rubber may, for example, indicate hardening), and other defects in essentially any dielectric materials. Different types of defects have distinguishable characteristics. The technique can also be successfully used on composite materials containing conductive components, but whose construction makes them overall nonconductors--for example, carbon fiber composites.