Some models and experimental results relative to light scattering from disordered solids are reviewed with particular attention to the low-frequency pan of the spectrum. Measurements of the low-frequency Raman scattering of superionic glasses (AgI borate glasses and AgI phosphate glasses at several AgI concentrations) and non-superionic glasses (samarium phosphate glasses containing 5 or 25% of Sm2O3) are presented. By comparing spectra at different temperatures, a low-frequency extra-scattering (ES) has been separated from the usual disorder-induced scattering, which, at low frequencies, is due to acoustic modes. We found that the ES is approximately Lorentzian in shape and its intensity strongly increases with rising temperature. Because these spectral features are found in both classes of samples (i.e. superionic and non-superionic glasses), it indicates that the origin of quasi-elastic scattering is not related to high ionic conductivity of the superionic systems as has been suggested but seems to be a general characteristic of disordered structures.