The consequence of creation of excitons in crystalline and amorphous SiO 2 , including localization of excitons by producing self-trapped excitons and formation of the E centers in amorphous SiO 2 as a result of exciton localization, is reviewed. It is emphasized that the luminescence emitted from the self-trapped excitons arises from the recombination of an electron-hole pair occupying the dangling bonds formed by the scission of an Si-O bond in the SiO 2 network. A new luminescence band induced from Si-implanted SiO 2 after annealing at 1100°C is interpreted in terms of electron-hole recombination at the interface between Si nanocrystals and SiO 2 . The nature of the complementary partner of the E center generated from an exciton is discussed. Experimental result which appear to show that the stability of the oxygen interstitials plays the major role in the annealing of the E centers are presented. It is also suggested that the local structural change of amorphous SiO 2 by ionizing radiation and amorphization under dense electronic excitation are related to the self-trapped excitons.