Partial differential equations are as old as calculus itself, occurring as examples in the papers of Newton and Leibniz. Since the beginning, they have been strongly linked to physics and other sciences. This volume presents an introduction to the classical theory, emphasizing along the way physical methods and physical interpretations. The book beings with a derivation of some of the classical partial differential equations, and a discussion of the limitations of the physical models upon which the derivations are based. The second chapter discusses the classical methods for studying PDEs, including the theory of distributions and the Petrovskij classification into elliptic, parabolic and hyperbolic equations. Among the more advanced methods discussed are spectral theory, the method of planar waves and the theory of semigroups. Every topic considered is placed in its present context in mathematical research, yet the book never loses sight of the non-specialist reader with an interest in physical applications.