A detailed review will be presented, summarizing available data on absorption of sound in homogeneous air and its main constituents as a function of frequency, pressure, temperature, and humidity. Disagreement exists between results of the various investigators, especially when different techniques are used. It is customary to compare experimentally observed absorption coefficients αexp with theoretical values αtheor, where the latter include the effects of viscosity, heat conduction, heat radiation, diffusion, and vibrational thermal relaxation. Observed values of r = αexp/αtheor vary from slightly less than unity up to about 4 for measurements in the laboratory and up to somewhat higher values for measurements out-of-doors. Even in carefully dried air, nitrogen, and oxygen, reported values of r vary over a wide range, depending on the measurement technique employed. It is interesting that in recent measurements, using direct source-to-receiver systems where the sound field is approximately a spherical wave, r values of about 1.2 are obtained. This approaches the result expected from classical kinetic theory for rotational relaxation.