In this Chapter an overview of the radio frequency microelectromechanical systems is given. Focus is oriented towards discussion on different micromachined passive circuits that includes transmission line, varactor, inductor, switch, and switch-based circuits followed by resonators. Fabrication of RF micromachined devices on different technology platforms are also discussed in this Chapter. Applications of RF micromachined devices and components are also discussed with respect to the super-heterodyne radio receiver and electronically steerable antenna for radar application. Finally, this Chapter discusses organisation of the book. © 2022, The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd.