This monograph presents the study of one-dimensional waves in inhomogenous and hereditary media (media with memory). A new, mathematically rigorous factorization method is applied, which yields either an exact or an asymptotic decomposition of the nonlinear wave equations under consideration into first order multipliers. A central result presented in this book is a factorization theorem for the simple wave equation with constant coefficients, which permits a view of Riemann invariants and simple waves from a different angle, and which is used to go over to wave equations with variable coefficients and memory.