Microwave nondestructive testing (NDT) techniques offer an alternative to other conventional NDT methods. Microwave/millimeter wave techniques (which roughly cover 0.3 to 300 GHz) are particularly useful for examination of dielectric composite materials because their low dielectric losses provide good depth of penetration of electromagnetic radiation in this band [1–4]. Conventional NDT techniques, such as high-frequency ultrasonic testing (UT), are associated with limitations, e.g., large variations in elastic properties of low-density composite materials, that make interpretation of complex UT signals difficult. Furthermore, the criticality of coupling a transducer to a sample surface limits the use of such techniques for on-line applications. High-frequency microwave (millimeter waves, 30–300 GHz) systems, when compared to their low-frequency counterparts, offer higher resolution and sensitivity to variations in dielectric properties of low-loss composites. Moreover, higher frequencies allow utilization of more compact systems, which are often important for practical applications.