Landau damping results from the loss of energy of collective motion to the excitation of individual particles. Well known for plasmons, it can also occur for phonons and photons, as is shown herein. For a coupled longitudinal plasmon-optical-phonon wave, it can be significant both near the phonon resonance and near the plasmon resonance. It is generally insignificant for coupled transverse photon-optical-phonon waves. These results are important for infrared spectroscopy and Raman scattering involving plasmons and optical phonons. © 1967 The American Physical Society.