This book presents a clear and systematic treatment of the mathematical methods of wave phenomena in solids and fluids that will be readily accessible to physicists, engineers, and applied mathematicians. The emphasis is on developing the necessary mathematical techniques and on showing how they can be effective in unifying the physics of wave propagation in a variety of physical settings: sound and shock waves in gases, water waves, stress waves in solids, etc. One of the unique features of this book is the treatment of the Hamilton-Jacobi theory in the setting of variational methods. This theory is fundamental to the treatment of the partial differential equations of wave propagation; it bridges the gap between classical mechanics and geometric optics and sets the scene for the development of the quantum mechanics. Another interesting feature of this book is that it organizes some of the more advanced material such as nonlinear theory and asymptotic expansion methods, now scattered in the literature, into a systematic presentation.