We present results of transmittance measurements on periodic layers ofAg/MgF2at optical and microwave frequencies. These one-dimensional, photonic band gap materials exhibit transparency bands at optical frequencies and a huge stop band that extends throughout the near-IR to microwave frequencies and beyond. A unique feature of these metal/dielectric photonic band gap (MD–PBG) materials is that the overall transmittance in the pass band may increase as more periods are deposited. The center frequency, width, and sharpness of the pass bands are adjustable and generally depend on the thickness of the layers and the number ofAg/MgF2periods. These simple periodic structures have applications as sensor and eye protection devices, heat reflecting windows, ultraviolet blocking films, and transparent electrodes for light emitting diodes and liquid crystal displays. ©1998 American Institute of Physics.