Electromagnetic slow wave structures (SWS) comprised of arrays of conductive obstacles are formed inside conductive parallel-plate waveguides These SWS may be formed using, for example, MEMS manufacturing processes at the wafer level on substrates including ceramic and silicon. An effective relative permittivity in the range of 15 to 40 may be obtained at millimeterwave frequencies. The SWS can be made absorptive by incorporating resistive losses in a plate of the PPW. Applications of these slow wave structures include delay lines and bootlace lens beamformers for microwave and millimeterwave antenna systems.