Ultrasonic inspection is a nondestructive method in which beams of high frequency acoustic energy are introduced into a material to detect surface and subsurface flaws, to measure the thickness of the material, and to measure the distance to a flaw. This chapter begins with an overview of ultrasonic flaw detectors, ultrasonic transducers, and search units and couplants. It then discusses the principles of operation, presentation, and interpretation of data of pulse echo and transmission methods. This is followed by sections providing information on general characteristics of ultrasonic waves and the factors influencing ultrasonic inspection. The advantages, disadvantages, and applications of ultrasonic inspection are finally compared with other methods of nondestructive inspection of metal parts.