The book introduces the reader to the physical phenomena associated with the spread of ultrasound, as well as their use in science and technology. Consists of more than 250 articles arranged in alphabetical order, which describe the main patterns characterizing the propagation of ultrasonic waves, consider the physical phenomena associated with them in the substance and highlight the areas of its practical use. In articles directly devoted to the use of ultrasonic methods in science, technology, medicine, emphasis is placed on identifying the physical basis of application and on the statement of principles of operation of ultrasonic devices.