An apparatus and a method are provided for electronically tuning cavity filters. A tunable cavity comprises at least two pieces of material, such as metal plates or metal traces, and MEMS circuitry interconnecting the pieces of material. Multiple tunable cavities can be combined to create a tunable cavity filter. In one embodiment, a waveguide cavity filter comprises a metal insert attached to a substrate. At least two pieces of material and MEMS circuitry reside within the cavities produced by the metal insert. The MEMS circuitry can be controlled to connect or disconnect the pieces of material, which alters the electric and magnetic fields inside the cavities. In another embodiment, a MEMS positioner inside the cavity filter can physically deform or move a piece of material within the cavity. By altering the electric and magnetic fields within the cavities the resonant frequency of the cavity filter can be tuned.