A new acoustic approach to medical imaging and diagnostics based on the remote evaluation of shear elasticity modulus of soft tissues is considered. The new method, called shear wave elasticity imaging, is based on the detection of shear waves remotely generated in tissues by radiation pressure of amplitude-modulated focused ultrasound. Remotely induced shear wave is fully attenuated within a short distance and the induced strain in the tissue can be extremely localized. By choosing appropriate temporal characteristics of the amplitude-modulated ultrasonic pulse, the volume of tissue involved in the mechanical excitation can be kept on the order of 1=A0 cm[sup 3], in contrast to other methods of elasticity imaging where the complete organ is subjected to shear stress. Consequently, evaluation of viscoelastic properties of tissue is greatly simplified since trivial boundary conditions can be assumed and an infinite medium model can be used to reconstruct the mechanical properties of tissue. Analytical equations describing the radiation force field induced by nonlinear focused ultrasound are derived and propagation of shear waves generated in tissues is considered. A possibility of deliberately exploiting the nonlinear effects to enhance the radiation pressure in a small area near the focus of the beam is shown.