The most important features of the physics of microcavity-embedded quantum wells are reviewed, with particular attention to the optical properties of exciton polaritons. The strong coupling regime, in which a finite polariton mode splitting appears (also called Rabi splitting), is studied. The problem of including both homogeneous and inhomogeneous broadening effects is addressed. The effects of exciton-phonon interaction and the polariton homogeneous broadening are briefly discussed. The influence of the quantum well in-plane disorder on the spectra of strong coupling samples is then investigated in order to understand the features of the inhomogeneous broadening of the polariton spectrum. Polaritons in low disorder heterostructures display a spectral narrowing, with respect to the ideal case in absence of disorder. This effect is markedly present in state-of-the-art samples. A simplified model of microcavity polaritons;which takes into account the quantum well interface roughness is presented. The results provide a satisfactory explanation of recently observed data in terms of multiple scatterings of polaritons on the quantum well disordered potential. (C) 1998 Elsevier Science B.V. All rights reserved.