A survey has been made of the various mechanisms that have been suggested for the absorption of sound in fluids. These can be classified into two groups,viz., (1) viscosity absorption and (2) relaxation absorption. In the second group are included the effects of heat radiation, heat conduction, thermal relaxation, structural relaxation, and a more general type of relaxation proposed here. This last mechanism includes both thermal and structural relaxation as special cases. For mechanisms of the second group the expression relating the absorption coefficient and the velocity with the frequency can be derived by starting with the first law of thermodynamics and arriving at an acoustical equation of state. This transformation can be made without any special assumption as to the nature of the substance. The equation of state permits one to make a simple comparison of the various approaches to the problem. A study of the velocity and absorption as a function of frequency shows important differences between the two classifications. Thus the assumptions that the additional absorption in water (beyond the classical viscosity contribution) is due to structural relaxation (Hall) or dilatation viscosity (Eckart and Liebermann) are not precisely equivalent.