Research on cell suspensions, single cells, macromolecular suspensions, and model (nonliving) systems reveal a variety of changes brought about by sound in the absence of gross-heating or transient (“collapse”-type) cavitation. Many of these phenomena are explainable in terms of acoustic streaming, radiation pressure, and other characteristics of ultrasound that arise from nonlinearity. Commonly, the sonic effect depends on nonuniformity in the sound field. Interestig results are obtained with techniques by which one can set up nonuniform vibration in the walls or membranes of individual cells. In ultrasonic beams acting on tissue, it is possible that variations may arise from gradients in the incident field, or from inhomogeneities in the tissue, which scatter sound. Gaseous pockets or bubbles would be especially effective as such inhomogeneities.