The risk of damage to paintings on wood (panel paintings) increases with the presence of cracks, delaminations, and their associated stress concentrations. Such flaws can originate and increase in size as a result of fluctuations in temperature and relative humidity, as well as shock and vibration. Many flaws are internal and therefore cannot be detected visually or by traditional testing techniques; thus it is diffucult, for example, to assess the risk that transportation poses to panel paintings. Air-coupled ultrasound can be used to assess the condition of panel paintings in a non-contact, non-intrusive manner; it provides information complementary to that given by radiography. It has been demonstrated that the ultrasonic system is clearly more suitable for detecting certain flaws, such as in-plane cracks and delaminations. The system enables measurements to be easily made of highly anisotropic and inhomogeneous materials such as wood. The ultrasonic system used in this study has a superior signal-to- noise ratio because it uses efficient transducers, low noise pre- amplifiers, and a phase-sensitive superheterodyne ultrasonic system which contains analog signal averaging and filtering components. The signal can be exploited to yeild both amplitude and phase information. Amplitude measurements detect delaminations and cracks, while phase measurements detect cracks. The ultrasonic system also incorporates a mechanical scanner to produce easily interpreted two-dimensional images of large areas of paintings to give clear indication of their condition. The results can be further enhanced by using image processing techniques. The ultrasonic system reliably detected cracks and delamination in the panel paintings Parental Admonition (a copy of the original), Sunset Scene, and Woman Gathering Yucca Plants. More applications have demonstrated the successful examination of a wide range of simulated panel paintings with different supports (white oak, tulip poplar, and hardboard), surfaces (gesso, gesso and linen, veneer, and paper), and flaws (cracks and delaminations). By using certain measurement techniques, the structural information in the ultrasonic scan was not obscured by the paint layer. The support layers were of thicknesses comparable to those of panel paintings.