Radiation pressure, or the pressure of ultrasonic radiation, which, in particular, appears in the form of steady ponderomotive forces acting on obstacles in the propagation path of an ultrasonic wave, can in a certain sense be regarded as a nonlinear effect. In a free ultrasonic field, ultrasonic radiation pressure also exists in the form of a constant component of the pressure. Radiation pressure is characteristic of any wave process, independent of its nature; it is related to the change in the magnitude of the momentum carried by the wave at an obstacle. The ponderomotive forces arising in this manner are weak. It is well known, for example, that very sensitive instruments are required to record the pressure of light. Ultrasonic radiation pressure is also a small quantity compared to the amplitude of the variable pressure in the ultrasonic wave. Nevertheless, the radiation effect follows directly from the linear equations of electrodynamics and the linearized equations of hydrodynamics. The nonlinearity of the exact equations of hydrodynamics adds corrections to the ultrasonic radiation pressure which are comparable in magnitude to the effect calculated in the first approximation, in contrast to the nonlinear corrections to other acoustic parameters, for example, the velocity of sound, the energy density,* etc., in which they appear as second and higher order quantities. These comparatively large corrections to the ultrasonic radiation pressure constitute the nonlinear effect. The difference between 126acoustic and electromagnetic radiation forces is also related to the fact that under the action of an ultrasonic wave the surface of an obstacle undergoes oscillations, which change the ultrasonic field. All this leads to the fact that calculations of the ultrasonic radiation pressure yield different results under different conditions: an infinite wave front, a bounded ultrasonic beam, an unbounded unperturbed medium, a closed ultrasonic field when the mass of the medium supporting the oscillations occur remains constant, the case of a “free” ultrasonic field, or the case when the radiation forces acting on an obstacle are calculated.