The acronym MEMS stands for microelectromechanical systems, which have lateral dimensions in the range of 1-1000 mm. MEMS are also sometimes referred to as microsystems technology (MST) and the fabrication techniques as micromachining. MEMS devices are fabricated using techniques that were originally developed for the microelectronics industry but that have been adapted and extended to construct a wide variety of devices that accomplish a large number of engineering functions. In particular, MEMS devices are used either as sensors or as actuators, and sometimes these elements are coupled together to produce a microsystem. MEMS may or may not contain moving parts. For reviews of the MEMS field, see for example Refs. [1-3].