Terahertz (THz) spectroscopy captures molecular and vibrational spectra over 10 GHz-10 THz. In the time domain, access to complex refractive indices makes it a versatile tool. THz time-domain spectroscopy (THz-TDS) has been used to study major glass families. The THz-TDS directly measures the THZ wave’s temporal electric field. Fourier transforms of the time-domain data yield the amplitude and phase of the wave pulse and the real and imaginary parts of the dielectric constant. An overview of the tool and its application to study selected model glass systems, e.g., silica, alkali borate, silicates, and tellurite and nonoxide systems will be presented. Our recent results show a linear correlation between refractive index measurements in visible and terahertz frequencies with systematic rare-earth ion inclusions in aluminosilicate glasses. In addition, our results demonstrate that THz-TDS can be utilized as a means of extending the detectable limits in refractive index measurements of disordered glass structures.