This document summarizes a review of 33 studies published between 1999 and 2013 on the role of metacognition in second/foreign language learning. The review found that metacognitive interventions can promote language performance but evidence is mixed on their effectiveness in enhancing metacognitive awareness/strategy use. Several factors were also found to affect L2 learners' metacognition. The review expands understanding of metacognition's role in language learning and has implications for teaching practices and directions for future research.