The possible effects of flow on airborne sound attenuation in ducts are discussed. The theoretical part results in a simple attenuation formula which considers the following: change of the wavelength due to convection of the sound field, change of the sound pressure at the wall caused by the flow profile, change of the characteristic absorber properties by nonlinear effects, and sound scattering by vortices. With porous absorbers another effect is caused by the different curvature of the phase plane at the boundary of the absorber. Measurements with a porous absorber and with damped Helmholtz resonators show reduction of the attenuation for sound propagation in the direction of the flow and an increase of the attenuation for sound propagation against the flow. With the help of pseudo-sound in flow and of partial waves in ducts with a periodic boundary structure, the sound amplification found in ducts coated with reactive absorbers can be explained by analogy to traveling wave tube amplification phenomena. This analogy was confirmed by measurements on resonant absorbers. (Author)