This book explores the importance of cross-linguistic similarity in foreign language learning. While linguists have primarily focussed upon differences between languages, learners strive to make use of any similarities to prior linguistic knowledge they can perceive. The role of positive transfer is emphasized as well as the essential differences between comprehension and production. In comprehension of related languages, cross-linguistic similarities are easily perceived while in comprehension of distant languages they are merely assumed. Production may be based on previous perception of similarities, but frequently similarities are here merely assumed. Initially, effective learning is based on quick establishment of cross-linguistic one-to-one relations between individual items. As learning progresses, the learner learns to modify such oversimplified relations. The book describes the ways in which transfer affects different areas of language, taking account of the differences between learning a language perceived to be similar and a language where few or no cross-linguistic similarities can be established.