I. INTRODUCTION A transducer is a device that changes certain physical values to other physical values. There are very many kinds of transducers related to elec­ trostatics. They include electrostatic sensors, electric energy converters, and electric actuators. Typical examples are acoustic transducers made of electrets, one of which is called an electret microphone and is widely used in the compact cassette tape recorder. Other transducers include electromechanical converters, electrostatic sensors, and capacitive con­ verters and are introduced in this chapter.