We review the work which has been carried out in the last decade on the linewidths of optical transitions in glasses. The techniques of fluorescence line narrowing, spectral hole burning and photon echoes have been used to obtain homogeneous linewidths (Γh) in the presence of strong inhomogeneous broadening in glasses. After a brief survey of experimental techniques, we review measurements made on rare-earth and transition-metal-doped glasses as well as organic molecules in glasses and polymers. In inorganic systems high-temperature (> 20 K) data show an approximately quadratic temperature dependence, while below this linear and T1.3 power laws have been found. For organic systems at low temperatures an almost universal T1.3 dependence of Γh is observed. Numerous theories have been proposed to explain these results and these are summarized. There is no real microscopic understanding of the origin of optical line broadening over a broad temperature range though several models containing important concepts have been proposed. More complete experimental data on few well-characterized systems are still needed as are theories to relate Γh to other glass properties. © 1987.