Amorphous SiO₂ is material with unique physical properties, including excellent optical transmission in the uv spectral range and a low coefficient of thermal expansion at cryogenic temperature. These properties make it an attractive candidate for excimer laser windows and as a surveillance mirror substrate. However, in both of these applications the silica may be exposed to substantial doses of ionizing radiation, either from the scattered electrons and x-rays from the laser pump or from the natural and/or enhanced on-orbit space radiation environment. This paper reports the effects of ionizing radiation on the uv transmission and density of SiO₂. Current mirror substrates are fabricated from fused natural quartz. However, the greater purity of synthetic silica leads to increased uv transmission and greater radiation resistance. Therefore, measurements have been made on a large suite of silica samples, including fused quartz, synthetic silica fabricated by flame hydrolysis, plasma fusion, or new techniques. In some cases the observed radiation sensitivity has been related to fabrication technique and impurity contents.