Linear and nonlinear optical properties in oxide and chalcogenide glasses have been studied comparatively. Applying a semiconductor concept to these glasses, we show that maximal nonlinear refractive-index at optical communication wavelengths is ~10 -4 cm 2 /GW, which can be obtained in materials with bandgap energy of ~1.6 eV. It is also shown for SiO 2 and As 2 S 3 that linear and nonlinear optical excitations induce different photostructural changes, which are attributable to different photo-electronic transition probabilities.