Publisher Summary This chapter presents the fundamentals of elastic wave propagation in porous media. Elastic waves are mechanical disturbances that propagate through a material. Such waves are able to travel over very long distances through the Earth and thus bring us information about parts of the formations that are otherwise inaccessible. Elastic waves in air and water are usually called acoustic waves or sound waves. These terms are often used about elastic waves in rocks too. The main focus of this chapter is the connection between rock acoustics and rock mechanics. Elastic waves in rocks propagate with a velocity that is given by elastic stiffness and the density of the rock. These parameters depend on other parameters such as porosity. Thus elastic waves also provide a method, by which specific formation parameters can be estimated in the field. Finally, this chapter gives some background to understand sonic wave propagation in boreholes and seismic waves through the subsurface.