This article reviews theories and research on the topic of the role of L2 code knowledge in adult second language acquisition. In the literature, this issue has been addressed from two main theoretical perspectives-cognitive psychology, which examines the internal processing mechanisms involved in learning and then in using language information for communication, and second language acquisition, which examines the effects of instruction that focuses on the L2 code on the learner's developing L2 proficiency. This analysis concludes that while the knowledge the learner has about the L2 code plays a limited role (planning and monitoring) in actual communication, it does play a significant role in the development of proficiency. Finally, instructional implications emerging from existing theory and research data are drawn.What role does the knowledge a second language learner has about the L2 code! play in developing proficiency, that is, the ability to use the L2 for communication purposes?Because of its important educational implications, this question has received considerable attention from theorists, researchers, and methodologists in the last decade. Traditionally, second language teaching methods were based on the assumption that learning a second language was equivalent to learning the L2 code and that, therefore, instruction that focused on the code (e.g., the learning of grammar rules in the Grammar Translation Method or the memorization of contextualized chunks of language in the AUdiolingual Method) was the optimum methodology. However, in light of our growing understanding of the range of competencies that underlie language proficiency2 and the realization that knowledge about the L2 does not necessarily indicate the ability to function effectively in communication, the value and role of knowledge about the L2 code in developing L2 proficiency has been questioned. This paper will review the theories and research data that have accumulated with a view to evaluating whether a consensus is emerging which has specifiable instructional implications.This issue of the role of formal knowledge about the L2 code has been investigated from two main perspectives: cognitive psychology and second language acquisition. First, in cognitive psychology, 50 TESL CANADA JOURNAL/REVUE TESL DU CANADA