The calculation of laser-induced radiative transition rates for electric-dipole transitions is reviewed. The effects of laser polarization can only be understood by considering the Zeeman structure of energy levels. In particular, the case of laser coupling of degenerate levels is considered by including the Zeeman structure of the upper and lower energy levels. The interaction of the laser radiation with the atomic or molecular resonance is formulated in terms of the density matrix. The electric-dipole matrix elements are written as the product of a scalar quantity, the reduced density matrix element, and a vector quantity that is a function of the angular momentum quantum numbers J and magnetic quantum numbers M for the upper and lower Zeeman states. The calculation of absorption and stimulated emission rates, absorption coefficients, and susceptibilities for the limiting case of an isotropic medium is reviewed. Polarization effects in laser-induced fluorescence are reviewed, and the theoretical framework for the analysis of these effects is developed. The calculation of polarization spectroscopy signals and of polarization effects in resonant four-wave mixing are discussed. The effects of hyperfine structure in diatomic molecules are discussed. (C) 2003 Elsevier Science Ltd. All rights reserved.