Children's knowledge about the occurrence of cognitive activities and their influence on a person's knowledge, beliefs, and behavior increases greatly during middle and late childhood. Knowledge of cognitive activities, such as memory, attention, reasoning, and the stream of consciousness, contributes to the development of children's social understanding, academic skills, understanding of science, and concepts of knowledge. In addition, children's understanding of cognition may provides a developmental bridge from young children's understanding of mental states to adolescents' and adults' epistemological reflection. This chapter proposes that knowledge of cognitive activities emerges through mutual influence among (a) children's conceptual knowledge of cognition, (b) children's phenomenological awareness of their own cognitive activities, and c) children's social experience. Research concerning children's knowledge of cognitive activities, cognitive monitoring, and the social influences on metacognitive development is reviewed, and directions for future research are identified.