Polycrystalline transparent infrared windows with good optical and mechanical properties are needed. Starting from nanopowders, and sintering to ultrafine grained dense materials, offers the possibility of tuning the final grain size in order to simultaneously optimize the optical and mechanical properties. We have developed a chemical synthesis process to produce nanoparticles of single phase oxides, such as MgO and Y2O3. The synthesis process has been scaled in-house to produce kilogram quantities per batch. The primary particle size of powders is in the 15-35nm range, and the aggregate size is in the 150-200 nm range. In addition to using conventional sintering techniques such as hot pressing, these nanopowders are being sintered to full density and a high degree of transparency using a novel microwave sintering process, which has the added advantages of uniformly and rapidly heating a green compact. In preliminary studies, fully dense MgO, with LiF as sintering aid, was synthesized with a final grain size in the 1-3 micron range. Effects of processing parameters, such as hot pressing temperature, pressure, and LiF content, on microstructure and transparency were studied.