This book traces the origins of modern varieties of Yiddish and presents evidence for the claim that, contrary to most accounts, Yiddish only developed into a separate language in the 15th century. Through a careful analysis of Yiddish phonology, morphology, orthography, and the Yiddish lexicon in all its varieties, Alexander Beider shows how what are commonly referred to as Eastern Yiddish and Western Yiddish have different ancestors. Specifically, he argues that the western branch is based on German dialects spoken in western Germany with some Old French influence, while the eastern branch has its origins in German dialects spoken in the modern-day Czech Republic with some Old Czech influence. The similarities between the two branches today are mainly a result of the close links between the underlying German dialects, and of the close contact between speakers. Following an introduction to the definition and classification of Yiddish and its dialects, chapters in the book investigate the German, Hebrew, Romance, and Slavic components of Yiddish, as well as the sound changes that have occurred in the various dialects. The book will be of interest to all those working in the areas of Yiddish and Jewish Studies in particular, and historical linguistics and history more generally.