Today, ultrasonic signal is being used for characterizing (detecting internal defects in) a variety of engineering structures, such as airplanes, buildings, nuclear power plants, pressure-vessels, and pipes, for inspecting human body parts, such as tumors, bones, and unborn fetuses. In spite of the ever increasing popularity and wide range of the use of this technology by researchers, medical technicians, factory workers, and engineers, we do not have any book that covers both the fundamentals and the advanced applications of this technology. Currently, books either concentrate on fundamental equation derivations, and thus are too theoretical, or consider only advanced applications, hoping that the readers have already gained the theoretical knowledge from another source.