The book places emphasis on both the mathematical significance and the strong physical background of wave equations. It presents the theory of wave equations in a unique way, different from the traditional descriptions provided by previous literature. The book is primarily focused on mathematical ideas and thoughts about wave equations. Starting from the modern theory of harmonic analysis, the book develops a few new tools in this field that are being used for better understanding the theory of mathematical physics underlying the well-posedness and scattering theory of wave and Klein-Gordon equations. Additionally, a significant part of this book discusses theories and methods, such as invariant and conservation laws, inward/outward energy methods, etc., that have never been covered by similar books in this field. Finally, the book briefly introduces recent developments in mathematical fields. It is specially designed for experts in mathematics and physics who deal with numerous applications of nonlinear waves in physics, engineering, biology, and other fields.