To advance an area of technology requires an understanding of the current state of the art. For the past thirty years the standard material for durable window applications has been single crystal sapphire. Sapphire has proven to be a difficult material to replace although many attempts have been made. A variety of polycrystalline materials have been developed and characterized. Also, engineered structures such aerodynamic outer shapes with corrective optics to replace hemispherical shapes have been studied. Recently polycrystalline materials with nanometer-sized grains have been developed. A review of midwave optical window material development is presented with an emphasis on optical properties.