Monolithic integrated circuit technology promises a practical means for realizing reliable and reproducible planar millimeter and submillimeter wave circuits. Planar circuits are fabricated through photolithographic techniques, which allow for the cost-effective production of intricate designs not possible with waveguide technology. Such circuits however, do not typically allow for post-fabrication optimization of performance, which can be a serious limitation. A mechanically adjustable planar tuning element suitable for integration in a variety of monolithic millimeter and submillimeter wave circuits has been developed. It is called a sliding planar backshort (SPB) and it can be fabricated as an integral part of a dielectric-coated coplanar transmission line. The SPB forms a movable RF short-circuit, which allows for the variation of the transmission line's electrical length. Measurements at 2 GHz have shown the return loss for the SPB to be better than -0.5 dB over a bandwidth of at least 50 %, on various coplanar transmission lines. A photolithographically fabricated SPB has been demonstrated in a planar quasi-optical 100 GHz detector circuit in which the response of a Schottky diode was successfully varied over a range of almost 14 dB. A technique for fabricating a micromechanical version of the SPB has also been developed. Two such SPB's were fabricated as integral parts of a quasi-optical 620 GHz monolithic integrated detector circuit, and used to vary the measured response over a range of almost 15 dB. Such tuning elements can be used for characterizing developmental circuits, and for optimizing the in-use performance of various millimeter and submillimeter wave integrated circuits.