The thermal conductivity, specific heat, and elastic moduli of silicon indicate that silicon is an attractive material for the substrates of large cryogenic mirrors. Elemental silicon is available in large single crystals as well as polycrystalline blanks. Silicon is already used extensively in optics for both high-index IR lenses and IR filter substrates. We report on cryogenic tests of silicon mirrors. Our samples show that on cycling from room temperature to 77 K, the dimensional stability is only slightly worse than that of fused silica, which is a known highly stable, cryogenic mirror substrate. The dimensional stability of silicon is much better than that of metal mirrors. The fabrication of silicon mirrors, including a 20 by 40 cm grating blank, for a high-resolution, IR spectrograph now under construction at NOAO is discussed.