For the full potential of ultrasonic energy to be realized as a medical diagnostic and therapeutic tool, and in order to assure safe and efficacious procedures, the mechanisms by which this form of energy interacts with living systems must be well understood. The processes by which ultrasound is absorbed in biological media are basic to the understanding of these interaction mechanisms. Until the mid 1950s, investigators devoted much attention to studies of the frequency dependence of the ultrasonic absorption coefficient in biological tissues, and it was found that the loss per cycle α/fwas largely constant, at least within the frequency range 0.2–10 MHz, and that as the complexity of the biological media increased, so did the ultrasonic absorption behavior. An additional early finding was that the protein content was largely responsible for the observed ultrasonic absorption, although contributions from the structural features of tissue were also apparent. The more recent studies have dealt with molecular models to explain the latter observations. To date the ultrasonic properties of only a few biologically important macromolecules in solution have been examined extensively, mainly globular proteins, and to a much lesser extent carbohydrates and nucleic acids. Three mechanisms have been suggested to explain the excess ultrasonic absorption in aqueous protein solutions, viz., proton transfer reaction, solvent‐solute interaction, and shear viscosity relaxation, and a fourth mechanism, helix‐coil transition, has been suggested in reference to polyaminoacids.