This article reviews research on working memory (WM) and its use in second language (L2) acquisition research. Recent developments in the model and issues surrounding the operationalization of the construct itself are presented, followed by a discussion of various methods of measuring WM. These methods include word and digit span tasks, reading, listening and speaking span tasks. We next outline the role proposed for WM in explaining individual differences in L2 learning processes and outcomes, including sentence processing, reading, speaking, lexical development and general proficiency. Key findings are that WM is not a unitary construct and that its role varies depending on the age of the L2 learners, the task and the linguistic domain. Some tests of WM may in fact be tests of differences in ability to attend to aspects of the L2. Future research will focus on matching tests of WM more closely with linguistic tasks and using more standardized, replicable measures of WM in new areas including writing in non-alphabetic scripts, instructional interventions and cognitive neuropsychology.