Metacognition, an awareness of one's own thought processes, plays a central role in explaining and describing the learning process. As it shapes learners' beliefs about and attitudes toward learning which in turn affect their behaviours, it directly relates to language learning and teaching as well. Thus, cautious teachers should understand and employ the available information on metacognition and then design curriculum and learning environments. This review article sets out to emphasize metacognition in language learning and teaching and to look into the critical ingredients of successful language learning and teaching. After an outline of metacognition and its components such as metacognitive knowledge, experiences and processes, the article dwells upon its relation to learning, beliefs about learning and language learning, and the need for metacognitive training. The article concludes that not only language teachers but also teacher training programmes should devote time to metacognitive training to better understand language learners and lead to self-directed, autonomous learners.