Nondestructive evaluation is a technique for determining the presence, type, and size of flaws in mechanical structures without destroying them. A description of various types of nondestructive evaluation methods is given in this article. Particular attention is paid to acoustic wave probing techniques. Major advances have been made in the development of the theory of acoustic wave scattering from flaws and in the development of methods for determining the nature of a flaw from the variation of the scattered acoustic wave amplitude with frequency and angle. Several new types of acoustic imaging methods have been developed; prime examples are the acoustic microscope and electronically focused and scanned acoustic wave imaging systems. New acoustic measurement techniques for determining the fields in materials and predicting their breaking stress are discussed, as is the development of new types of eddy current techniques and electromagnetic transducers.