Much of the significant damage that may be incurred by graphite-epoxy composite materials results in relatively large discrete damage sites such as delaminations. In those instances, the damage is easily and reliably detected using conventional ultrasonic scanning techniques. However, several mechanisms also exist which do not result in the creation of large discrete damage sites, but result in damage which is distributed throughout the material. These mechanisms include the production of porosity due to poor curing procedures, damage resulting from fatigue or overheating of the composite structure in service, and water ingress caused by prolonged exposure of the material. Of these, the detection of porosity during manufacture has received the most attention, although the assessment of damage from the other sources is also important.