The two properties of a medium that enable it to conduct acoustic waves are (1) its mass and (2) its elastic properties. The simple presentation in this chapter of the manner in which each of these two properties infl uence acoustic wave propagation is intended to assist the reader in understanding how to apply the principles and data presented in later chapters. The temptation to offer a more extensive treatise has been suppressed in the interest of providing a useful reference book, rather than a textbook.