The fabrication and use of composites in thick sections create special needs for flaw detection and characterization which cannot be met by conventional nondestructive evaluation (NDE). Thick composite sections are susceptible to a variety of fabrication defects and in-service damage. Typical fabrication defects include matrix cracking, porosity, delaminations, fiber misalignment and waviness, fiber fractures, and nonuniform matrix distribution (fiber volume ratio). A consequence of fabrication defects is the variation of mechanical properties through the thickness and the buildup of residual stresses. In-service defects result from environmental factors, such as thermal gradients, and mechanical loading, such as impact.