Recent developments in the theory of impurity centers are reviewed. Particular attention is devoted to the way this theory can be exploited in the quantitative analysis of structured optical spectra through the use of formulas relating the absorption band shape to the shape of the fluorescence band. Analysis of departures from mirror-image relation between these two spectra can be used to deduce the parameters of the electron-phonon coupling from the shape of the structured bands. Topics discussed include zerophonon lines, the dependence of their shape and position on temperature, departures from mirror-image relation for both phonon wings and the vibronic absorption and fluorescence spectra, the energy gap between the absorption and fluorescence spectra of certain molecules, and the nature of excimer emission in solids.