This introduction to Applied Linguistics focuses on second language acquisition. It discusses different theoretical approaches from an interdisciplinary perspective, and deals with the main areas of research, such as individual differences, the role of instruction, cross-linguistic influence or input and interaction. It also includes a chapter on the acquisition of English as a third language. This volume is designed as a guide for students of Applied Linguistics, includes practical activities and an annotated bibliography at each chapter, plus additional sources for the study of second language acquisition such as dictionaries and internet links.