Dating back to the early 80s, the FL/SL literature on learner strategies and on self-directed language learning documents an ongoing recognition of the need to help language learners reflect upon and refine their beliefs and knowledge about learning, i.e. their metacognitive knowledge. To date, however, this literature has not been explicit about the function of this knowledge in language learning. This article reviews selected theoretical and research literature on metacognition to address this lack. It argues that insights provided by the review can enhance our understanding of those approaches to second language acquisition which assign an active role to the learner, and concludes with a consideration of practical implications for foreign and second language instruction.