Corrosion is one of the most important factors limiting life-extension of aircraft in both commercial and military fleets. Non-destructive methods for characterizing damage caused by hidden corrosion in layered structures such as aircraft lap-splices are, consequently, a high priority for commercial airlines and the Department of Defense. A considerable effort is underway to develop new eddy-current techniques for detecting and characterizing hidden corrosion. Eddy-currents have the advantage of penetrating into subsurface layers and therefore being sensitive to their condition, whether or not the layers are mechanically bonded. In contrast, ultrasonic techniques require a mechanical bond between layers for the ultrasonic energy to penetrate to the second or third layers. Both time-domain [1,2] (pulsed) and frequency-domain [3,4] (continuous wave) methods are under development.