This paper seeks to update previous reviews of the effects of ionizing radiation on amorphous insulators. The electronic and atomic structures of radiation-induced defect centers (trapped electrons or holes) are discussed primarily in terms of data deriving from the technique of electron spin resonance. Emphasis is placed on important fiber-optic materials (pure and doped fused silicas) and attention is directed to the problem of relating radiation-induced optical absorption bands to specific defect structures.