Historical sociolinguistics is a comparatively new area of research, investigating difficult questions about language varieties and choices in speech and writing. Jewish historical sociolinguistics is rich in unanswered questions: when does a language become 'Jewish'? What was the origin of Yiddish? How much Hebrew did the average Jew know over the centuries? How was Hebrew re-established as a vernacular and a dominant language? This book explores these and other questions, and shows the extent of scholarly disagreement over the answers.