This document summarizes theories of second language acquisition. It discusses four focus areas: transfer, where learners apply first language features to the second language; interlanguage, the language system learners develop between the first and second languages; motivation, which is important for learning and can be instrumental or integrative; and input and output, the importance of receiving input and producing output in the second language. It also briefly introduces the concepts of communicative competence, involving accurate, appropriate, and flexible language use, and applied linguistics, which deals with practical language issues.