New, more durable windows and domes will be required for future 3-5 micron IR systems. Various oxide and oxynitride crystalline materials are candidates for these new requirements, including sapphire, spinel, ALON, yttria, and MgO. These materials are compared with respect to optical properties, durability, and fabrication costs. Two extrinsic properties, thermal conductivity and fracture strength, have strong effects on thermal shock resistance. Birefringent polycrystalline materials will not have adequate optical resolution for future systems.