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Summary form only given. After the first observation of resonance scattering from excitons in quantum wells about 1980, this topic has received much interest and has given rise to a hot debate on the origin of this scattering process. Recently, the determination of the temporal behaviour and the coherence properties of this scattering has initiated revival of interferometric techniques in semiconductor optics. Finally, the observation of speckles in the angular distribution proved that this scattering indeed originates from exciton states localized by disorder. However, complementary information on the nature of the excitons and the interplay between scattering and luminescence can be obtained from high resolution cw spectroscopy, having the advantage that only a small fraction of the states within the homogeneous linewidth is excited. However yhe polarization behaviour is quite different for the coherent Rayleigh component and the luminescence, depending on the orientation, splitting and relaxation of the localized states. The experimental results for a 20 nm wide GaAs quantum well clearly confirm this prediction.