Sonochemistry, or the application of power ultrasound to chemical processes, is a technology that has undergone very intensive research and development in the last 15-20 years, with respect to both its range of application and the methods employed for its implementation at industrial manufacturing scales. In a series of two papers, the authors review progress of the application of ultrasound to chemistry and chemical processing, and the development of equipment and scale-up methods, This first paper focuses on chemistry and also corers mechanisms of action of ultrasound and its in situ detection and measurement, Part 2 will deal with dhe application of ultrasound to crystallization processes (sonocrystallization) and with scale-up and equipment development.