The term second language acquisition (SLA) refers to the processes through which someone acquires one or more second or foreign languages. SLA researchers look at acquisition in naturalistic contexts (where learners pick up the language informally through interacting in the language) and in classroom settings. Researchers are interested in both product (the language used by learners at different stages in the acquisition process) and process (the mental process and
environmental factors that influence the acquisition process). In this chapter I trace the development of SLA from its origins in contrastive analysis. This is followed by a selective review of
research, focusing on product-oriented studies of stages that learners pass through as they acquire another language, as well as investigations into the processes underlying acquisition. The practical implications of research are then discussed, followed by a review of current and future trends and directions.