Аннотация:We review the propagation of optical and hydrodynamic perturbations in a gas having nonequilibrium internal degrees of freedom. We discuss the nonequilibrium processes that lead to variation of the refractive index and self-focusing via the effect of kinetic cooling and nonequilibrium vibrational excitation of molecules. We analyze the propagation of small and nonlinear perturbations, as well as shock waves, in nonequilibrium gases.