A radio-frequency microelectromechanical system (RF MEMS) is a microelectromechanical system with electronic components comprising moving sub-millimeter-sized parts that provide radio-frequency (RF) functionality. RF functionality can be implemented using a variety of RF technologies. Besides RF MEMS technology, III-V compound semiconductor (GaAs, GaN, InP, InSb), ferrite, ferroelectric, silicon-based semiconductor (RF CMOS, SiC and SiGe), and vacuum tube technology are available to the RF designer. Each of the RF technologies offers a distinct trade-off between cost, frequency, gain, large-scale integration, lifetime, linearity, noise figure, packaging, power handling, power consumption, reliability, ruggedness, size, supply voltage, switching time and weight. Components There are various types of RF MEMS components, such as CMOS integrable RF MEMS resonators and self-sustained oscillators with small form factor and low phase noise, RF MEMS tunable inductors, and RF MEMS switches, switched capacitors and varactors. Switches, switched capacitors and varactors The components discussed in this article are based on RF MEMS switches, switched capacitors and varactors. These components can be used instead of FET and HEMT switches (FET and HEMT transistors in common gate configuration), and PIN diodes. RF MEMS switches, switched capacitors and varactors are classified by actuation method (electrostatic, electrothermal, magnetostatic, piezoelectric), by axis of deflection (lateral, vertical), by circuit configuration (series, shunt), by clamp configuration (cantilever, fixed-fixed beam), or by contact interface (capacitive, ohmic). Electrostatically actuated RF MEMS components offer low insertion loss and high isolation, linearity, power handling and Q factor, do not consume power, but require a high control voltage and hermetic single-chip packaging (thin film capping, LCP or LTCC packaging) or wafer-level packaging (anodic or glass frit wafer bonding). RF MEMS switches were pioneered by IBM Research Laboratory, San Jose, CA, Hughes Research Laboratories, Malibu, CA, Northeastern University in cooperation with Analog Devices, Boston, MA, Raytheon, Dallas, TX, and Rockwell Science, Thousand Oaks, CA. A capacitive fixed-fixed beam RF MEMS switch, as shown in Fig. 1(a), is in essence a micro-machined capacitor with a moving top electrode, which is the beam. It is generally connected in shunt with the transmission line and used in X- to W-band (77 GHz and 94 GHz) RF MEMS components. An ohmic cantilever RF MEMS switch, as shown in Fig. 1(b), is capacitive in the up-state, but makes an ohmic contact in the down-state. It is generally connected in series with the transmission line and is used in DC to the Ka-band components. From an electromechanical perspective, the components