Written for students encountering the topic for the first time, this is a clear and practical introduction to second language acquisition (SLA). Using non-technical language, it explains how a second language is acquired; what the learner of a second language needs to know; and why some learners are more successful than others. This new edition of Muriel Saville-Troike's bestselling textbook introduces in a step-by-step fashion a range of fundamental concepts, such as SLA in adults and children, in formal and informal learning contexts and in diverse socio-cultural settings. Taking an interdisciplinary approach, it encourages students to consider SLA from linguistic, psychological and social perspectives. Providing a solid foundation in SLA, this book has become the leading introduction to the field for students of linguistics, psychology and education, and trainee language teachers.