Abstract: Ultrasonic transducers convert electric signals into acoustic signals in the ultrasonic frequency range, or vice versa. They are key devices in ultrasonic technology and their performance determines the effectiveness and uses of ultrasonic technology. Because of their diverse applications, there are many types of ultrasonic transducers. In this paper, various transducers for different applications are described, and their theory, design and performance requirements are analyzed. Their characterization is also outlined, and development trends are analyzed.