Unlike liquids and gases, which in practice exhibit only bulk elasticity, solids also exhibit shear elasticity (“elasticity of shape”). The presence of shear elasticity, which, as always, we shall first assume to be ideal, leads to the fact that in a solid, together with the longitudinal elastic waves studied in the preceding chapters, shear deformation waves can also propagate in the form of so-called transverse (shear) waves. The laws of propagation of both types of waves in an infinite isotropic solid are exactly the same as the laws studied in the preceding chapters, which concerned ideal media with ideal elasticity, so that most of the previous results are equally valid for transverse waves. The distinctive features of the propagation of elastic waves in isotropic solids are, however, manifested primarily at the boundaries as different types of surface waves, mixed deformations, transformation of waves with reflection from boundaries, etc. For this reason, in this chapter, after deriving and analyzing the wave equation for an isotropic solid, we shall study only the basic problems concerning the propagation of ultrasonic waves, as well as some of the distinctive features of the propagation of finite-amplitude ultrasonic waves, in infinite solids.