The theory of the effect of optical saturation upon the reflectance of a plane slab (irradiated by a linearly polarized monochromatic electromagnetic wave of constant amplitude at normal incidence) is developed. The absorbing centers within the slab are described by means of the density matrix, and the electromagnetic field is described by Maxwell's equations. For simplicity's sake, it is assumed that only two quantum states (characterized by a transition dipole moment &mgr; and relaxation timesT1andT2) at exact resonance with the electromagnetic wave are necessary to describe the absorption process. The coupling between waves travelling in opposite directions is provided by Fresnel reflection at the boundaries of the slab. Only the steady‐state solutions are found.