The stress level in a material is usually measured with an electrical resistance strain gage attached permanently to the object. Such an approach actually yields only the change in stress when a load is applied and thus gives no information on the state of residual or thermal stresses that may be present in the unloaded material. X-ray methods can be made to yield absolute stress levels but they are time consuming and only give values characteristic of the first few microns of the surface layer. Elsewhere in this volume, several articles {1} can be found that describe magnetic methods that infer stress in steel from the magnetic field dependence of certain magnetic properties. Not only are these methods applicable only to steel but they suffer from the fact that they must be calibrated for the specific alloy being used and are based on experimentally established correlations between the stress and the particular quantity being measured. Ultrasonic techniques, on the other hand, are generally applicable to any material and are much less susceptible to uncertainties arising from the empirical tests used to calibrate them. However, like the electrical resistance strain gage, they are normally used to measure only relative changes in stress because the rolling textures that are often present in commercial structural materials introduce effects that cannot be distinguished from residual or thermal stresses.