Originally published in 1985, this monograph describes the interaction of radiation with plasma. Using an approach that is particularly relevant to the interpretation of data from laboratory plasmas or stellar atmospheres, the author sets out the physics and mathematics of the interaction of photons with atoms, molecules, ions and electrons. The emphasis throughout is on relating the formal mathematics to the real world of observable properties and interpretation. The equation of radiative transfer for a two-level atom is solved exactly by two distinct methods. Techniques for solving more realistic problems are then presented. This leads to the main thrust of the book which gives a detailed analysis of the matter - radiation interaction.