The ultrasonic holographic principle is described and several methods of producing ultrasonic holograms are outlined. A recently developed Holographic Ultrasonic Imager using a liquid-surface detector is described in detail. The instrument permits real-time imaging of test objects as they are moved through the ultrasonic field of view. Advantages of the instrument are discussed. The results of typical nondestructive test applications in metals, plastics, ceramics and composites are presented.