The interaction between quantum well excitons andcavity photons in semiconductor microcavities in the strongcoupling regime results in mixed 2D exciton–photon states,called exciton polaritons. The behavior of a dense polaritonsystem is of particular interest due to the fact that theseparticles have integer spin and, hence, obey Bose–Einsteinstatistics. Drastic nonlinearities have been observed both in thelow polariton (LP) emission intensity and polarization in thecase of the resonant excitation into the LP branch undercondition that 2ℏω(kex) = ℏω(k = 0) + ℏω(2kex). The experiments have shown a very strong final state stimulation of a twopolariton scattering due to the bosonic nature of the polaritons.The filling of k = 0 LP state significantly exceeding 1 has beenrealized under continuous excitation at T = 1:8 K. Thedependence of the effect on the polarization of photoexcitedlight and temperature is discussed.