This article provides an overview of our work on the nature of metacognitive knowledge, its relationship to learning through inquiry, and technologies that can be used to foster and assess its development in classrooms as students engage in collaborative inquiry. To illustrate our theoretical ideas, we present examples from our Inquiry Island software. It provides learners with advisors, who contain knowledge, advice, and tools aimed at supporting students' metacognitive development in the context of doing inquiry projects. Our pedagogical approach includes having young learners take on the roles of various cognitive, social, and metacognitive advisors as a way of enacting and internalizing the forms of expertise they represent. We describe a sequence of learning activities and indicate how students respond to them, using examples and findings from a 5th-grade class. Our work shows how such learning tools and activities can foster the development of metacognitive knowledge and skills needed for collaborative inquiry and reflective learning.