The name “micromachined ultrasonic transducer (MUT)” is used for devices that are made using microfabrication techniques to generate and detect ultrasound. Although this general term also applies to transducers made by shaping bulk piezoelectric materials using micromachining techniques, the more specific definition refers to devices with tiny membranes that are driven by either capacitive or piezoelectric actuation. The advent of both capacitive and piezoelectric micromachined ultrasonic transducers, cMUTs and pMUTs, respectively, has been made possible by advances in integrated circuit fabrication technology. This article focuses particularly on cMUTs.