This chapter aims at depicting the development of metacognitive knowledge from early childhood through early adolescence. Children’s ability to reflect on their mental states develops rapidly in early childhood, beginning around the age of three. Thus the chapter first discusses important precursors of metacognitive knowledge, in particular, the correct use of mental verbs and the development of a “theory of mind.” Subsequently, it highlights essential stages in the acquisition of declarative metacognitive knowledge, with an emphasis on knowledge about the effectiveness of different strategies. Next, it discusses how and when important aspects of procedural metamemory, that is, monitoring and self-control, develop in childhood and how they interact. It then discusses developmental trends in the relationship between metamemory and memory in more detail. Finally, it presents attempts to assess and foster metacognitive skills in applied settings and discusses possible future directions of research on the development of metacognition.