During the past few decades, waveguide switches have been the number one candidates for the majority of high-frequency low-loss applications. The switches employ a mechanical rotary motor which makes or breaks the waveguide RF path to turn the switch ON or OFF. However, the associated disadvantages with these switches such as their size and weight overshadow their excellent RF performance. Thus, miniaturization methods are investigated to potentially reduce the size and weight of these devices. By the rapid advancement of Micro-electro-mechanical Systems (MEMS) technology, employing MEMS elements in RF and millimeter-wave waveguide devices is being explored to potentially remove the heavy and bulky mechanical motors. In this project, realization of monolithic MEMS waveguide structures is being investigated. As the result, monolithic waveguide structures and their transition to planar transmission line on a single substrate is studied first. Then the MEMS components are incorporated inside the waveguide channel to replace the mechanical motor. By the electrostatic actuation of the MEMS components, the waveguide switch can be turned ON of OFF. The concept can be generalized to implement a whole category of on-wafer waveguide devices such as multiport switches, switch matrices, filters and antennas.