Metacognition involves monitoring and regulating thought processes to make sure they are working as effectively as possible (Brown, 1987; Flavell, 1976; Winne, 2001). Good teachers are highly metacognitive (Lin, Schwartz, & Hatano, 2005; Sherin, 2002). They reflect on their expertise and instruction, and they refine their pedagogy accordingly. Good teachers are also metacognitive in a less conventional sense of the term. They monitor student understanding and they regulate the processes that the students use to learn and solve problems (Shulman, 1987). Thus, good teachers apply metacognition to other people’s thoughts. The proposal of this chapter is that asking children to teach and apply metacognition to others can help them learn both content knowledge and metacognitive skills. A strong version of this proposal, consistent with Vygostky (1978), would be that metacognition develops first on the external plane by monitoring others, and then turns inward to self-monitoring. The chapter does not test this claim. Instead, it shows that having students teach a specially designed computer agent leads to metacognitive behaviors that increase content learning and hint at improving metacognition more generally.