A theoretical description is given for the propagation of longitudinal plane waves of large lateral extent in solids, for waves of plastic as well as elastic strain. The importance of the hydrostatic compressibility in determining the nature of the waves is brought out. The results are illustrated by a computation of the velocities of propagation of such waves in 24S-T aluminum alloy, and by a computation of the propagation of a pressure pulse of short duration through a flat plate. The later results illustrate the attenuation and change in shape of the pressure pulse during its propagation. The effects of the pressure dependence of the compressibility, the temperature changes accompanying compression, and the time and rate dependence of the mechanical properties, upon the propagation of such waves are discussed qualitatively.