One of the most important characteristics of glass as a material is the optical transparency over a wide spectral range. In this paper, optical properties related to this are reviewed. We describe the characteristics of the optical transmission spectra of compounds, i.e., isolators or semiconductors, including glasses, and the factors that determine the wavelengths of the absorption edges at the short and long wavelength sides. Next, we discuss the scattering and refraction phenomena in the transparent region in glasses and finally explain coloration briefly. We also touch on some recent research results.