We present a comparison between the semiclassical and the full quantum descriptions of optical properties of quantum wells (QWs) in semiconductor microcavities. The semiclassical theory, which is based on a nonlocal treatment of the optical response of a single QW, allows one to calculate reflectivity, transmission, and absorption curves. The quantum theory, which is developed in a quasi-mode approach for the coupling of-the closed-cavity modes to the outside radiation continuum, provides a description of photoluminescence processes. The frequency-dependence of the phase of the mirror reflection coefficient has an important role in both theories: Numerical results show that the mixed mode (polariton) dispersion and the Rabi splitting calculated from semiclassical and quantum theories coincide rather well.