This chapter introduces the central issues and concepts in the fields of bilingualism, contact linguistics, and second language acquisition, with an eye to identifying generalizations and key distinctions. For example, although authors use terms like transfer, interference, bilinguals, etc., across fields, these sometimes have the same meaning and other times depend on the subfield of linguistics in which they are employed. While there is a good deal of overlap between these fields, the current chapter provides an overview of studies and findings related to these concepts in order to help orient the reader to the hypotheses, methodologies, and discussions contained in the present volume.