The science of producing and transmitting sound waves in materials has become a flourishing art with many practical applications. Among the low-amplitude uses are underwater sound transmission for locating submarines, measuring the depth and detail of ocean bottoms, flaw detection in materials, delay lines for storing informtion and for performing processing calculations, and many medical applications such as locating cancers and other imperfections in the human body. Ultrasound waves have ben used in many physical investigations. When tho amplitude of the sound wave becomes large, some non-linear effects occur such as the produotion of cavities in liquids and fatigue In solids. These give rise to another set of applications such as ultrasonic cleaning, emulsification of liquids, machining of materials, and tests for fatigue in materials - particularly when a number of cycles is required. Biological applications such as the destruction of bacteria and the use of foamed ultrasound as a surgical knife are also possible. It is the purpose of this paper to provide the historical background leadlng to these applications and to discuss some of them in detail.