This paper discusses state-of-the-art ultrasonic testing as used to inspect and evaluate parts and materials for aerospace systems. The paper is divided into four sections: purpose of testing; basic requirements for application, such as equipment, reference standards, and acceptance criteria; standard applications, such as wrought metal products, tubing and pipe welded assemblies, thickness and corrosion, fiber-reinforced composites, adhesive-bonded assemblies, brazed and diffusion-bonded assemblies, aircraft maintenance inspection, attenuation, and grain-boundary reflections; and research and development applications, such as the liquid-level detector, titanium hydride detector, measurement of applied stress, ultrasonic extensiometer, ultrasonic modulus versus mechanical properties for fiber composites, ultrasonic spectroscopy, acoustic holography, and critical angle reflectivity.