This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book discusses the field of musical semiotics using the work of several distinguished theorists and then demonstrates how Olivier Messiaen established a rudimentary sign system at the start of his career with his innovations in rhythm, scales, colour, and the use of plainsong. It shows that how the sign system is expanded and refined in the 1940s. The book traces the history of musical cryptography, noting in particular developments in France in the twentieth-century, especially the work of Francois-Bernard Mache, whose compositions from the 1950s to the 1980s went well beyond Messiaen's linguistic experiments. It focuses on technical analysis of the structure of the 'langage', and deals with issues of acquisition, perception and retention. The book also discusses the problems associated with meaning in music, and how they relate to Olivier Messiaen's sign system.