Ultrasonic methods for nondestructive testing (NDT) and material characterization are perhaps the most versatile of the testing techniques available to industry. Ultrasonic NDT, therefore, provides one of the largest non-medical areas for applications of low-intensity ultrasonic energy. This chapter discusses ultrasonic inspection/measurement methods, factors that affect the capabilities to detect defects, and some basic aspects of ultrasonic NDT instrumentation. Resonance methods have a long history, and they been used to measure the thickness of plates, pipe, and tank/vessel walls and for the detection of delaminations when only one surface is available. Ultrasonic pulse methods of NDT are used much more extensively than resonance methods in industry. Acoustic emission is the basis of an important class of NDT methods. Acoustic emission is the spontaneous emission of sound pulses from materials subjected to external stress as a result of sudden relaxation of stresses within the material.