This thesis presents the development of a Micro-Electro-Mechanical System (MEMS) monolithic in-plane tunable optical filter in both Indium Phosphide and Silicon. By placing one mirror of a waveguide-based Fabry-Perot interferometer on an electrostatically-actuated beam, a tunable filter is constructed. This is the first demonstration of a waveguide-coupled MEMS tunable optical filter in any material system. Filters with a tuning range of 40 nm from a wavelength 1550 nm with a linewidth of 35 nm are demonstrated. Future work will concentrate on improving the filter's optical characteristics, limited by etch-induced facet roughness, and integration with active photonic devices.