Nondestructive testing in the broad sense refers to methods whereby internal characteristics of solid structures can be examined without permanently affecting the structure. Thus, parts that prove to be satisfactory under specified test conditions are not degraded by the test procedures. We are concerned primarily with the question “Will the structure satisfactorily perform its intended function throughout its service life?” Implications of this question as related to the interpretation of the results of nondestructive test methods will be discussed. Elastic waves are presently employed to detect gross internal mechanical defects such as voids, fractures, and impurities. Many internal characteristics that bear on the question of structural performance are not tested by ultrasonic techniques. These include such properties as grain structure and size, variation in elastic moduli, variation in chemical composition, presence of permanent strains, structure of domains, depth of surface treatment produced by hardening procedures, effects of radiation, etc. A discussion of the importance of these additional internal characteristics will be given, and the applicability of ultrasonic procedures to the measurement of these properties will be treated.