What are medieval polyphony and song? Who composed this music, sang it, and wrote it down? Where and when did the different kinds of polyphony and song originate, and under what circumstances were they composed and performed? This chapter provides definitions of the key terms and sets out the scope of the book. It explains how we have grouped the material into chapters based on geographical locations, and the rationale for this organization. We then give a broad, outline history of the medieval period (or ‘Middle Ages’) to help readers navigate the major events, characters, and places that helped to shape this long historical period. A timeline of key musical and historical events of the Middle Ages concludes the chapter, giving a helpful point of reference for readers to understand the overlapping chronologies of different kinds of medieval polyphony and song, and their historical contexts.