It has frequently been argued that there are significant parallels between elements of ‘Germanic’ and ‘Celtic’ religious history, stretching from antiquity to the Middle Ages, which indicate Celtic influences within Germanic religion. Elements of Germanic religion and mythology for which such influence has been proposed range from the Batavian seer Veleda to the Valkyries, from the cult of the Matres to the figure of Heimdall, from the myth of the death of Balder to the motif of the Everlasting Battle; the connecting lines that have been drawn extend to central aspects of Germanic religion in antiquity and nearly the whole of medieval Norse mythology. Yet few such proposals have ever been critically discussed, and even fewer attempts have been made to come to grips with the methodological problems raised by cultural contact research in Germanic religious history more generally. The present book is an attempt to address this situation: placing a strong focus on questions of methodology, it offers not only the first detailed summary, but also the first critical analysis of the state of scholarship on the question of Celtic influences in Germanic religious history.