More than two billion microphones are produced annually. The electret microphone, invented by Sessler and West in the early 1960s, makes up the bulk of this production. With the desire for more functionality as well as smaller size, microelectromechanical systems (MEMS) microphones are now being proposed to become the microphone of choice for consumer audio applications. There are two major application areas that are driving the interest in MEMS microphones: hearing aids, where size and integration with digital signal processing are important, and consumer devices, where there is interest in reducing costs by integrating a complete systems solution on an integrated circuit. This paper describes some fundamental MEMS microphone design issues. An all-surface machined MEMS microphone that was built at Bell Labs is also described. © 2005 Lucent Technologies Inc.