The exciton-polariton states of quantum wells embedded in planar semiconductor microcavities are usually described by assuming in-plane wave vector conservation. In this work we consider the two main k(parallel to)-scattering mechanisms, namely the polariton-phonon scattering and the in-plane structural disorder of the quantum well. We calculate the homogeneous and inhomogeneous broadening of the polariton levels and show that a detailed model of the scattering processes is essential for a theoretical derivation of recently observed features of the linear spectra.