A density-matrix description has been developed to treat relaxation (decoherence) phenomena during resonant and non-resonant radiative transitions of quantized electronic systems, including many-electron atoms and quantum-confinement systems (e. g., semiconductor microstructures). Radiative and collisional relaxation phenomena have been treated using Liouville-space proj ecti on-operator techniques. Both time-independent (resolvent-operator) and time-dependent (equation-of-motion) formulations have been developed. The self-energy operators that occur in these formulations can provide the fundamental basis for a self-consistent determination of the non-equilibrium and coherent electronic-state kinetics together with the homogeneous spectral-line shapes. This density-matrix description can be adapted for the computer simulation of electromagnetic processes. From first-principles electronic-structure calculations or from semi-empirical approaches, the parameters describing the elementary collisional and radiative interactions can be evaluated and organized into the basic data set for the application of the density-matrix description. The final product is a theoretical prediction for the linear or non-linear optical absorption or emission spectrum corresponding to a given set of values for the appropriate physical variables, such as temperatures, densities, and electric or magnetic field strengths.