RF MEMS are small mechanical devices fabricated by photolithographic processes, which are used for elemental signal processing functions in rf and microwave frequency circuits. The most common RF MEMS control component is a microwave transmission line switch, currently under development for applications requiring low insertion loss, high linearity, moderate switching speeds, and low to moderate power. The RF MEMS switch promises integration onto a variety of substrates, including substrates bearing active semiconductor devices. This paper will discuss some aspects of the operation of RF MEMS switches and how they may be inserted into microwave circuitry to best advantage.