In the ThinkerTools Inquiry Project, researchers and teachers collaborated to create a computer-enhanced, middle-school, science curriculum that enables students and teachers to learn about the processes of scientific inquiry and modeling as they construct a theory of force and motion. Our primary objective has been to create an instructional approach and materials that make scientific inquiry accessible to a wide range of students, including lower-achieving and younger students. Our hypothesis is that this objective can be achieved by scaffolding the development of metacognitive knowledge and skills. Toward this end, the curriculum centers on a metacognitive model of research, called the Inquiry Cycle, and a metacognitive process, called Reflective Assessment, in which students reflect on their inquiry. In our approach, the class functions as a research community. Students propose competing theories. They then test their theories by working in groups to design and carry out experiments using both computer models and real-world materials. Finally, they come together to compare their findings and to try to reach a consensus about physical laws, models, and theories that best account for their results. This process is repeated as the students tackle new research questions that foster the evolution of their theories of force and motion. In this article, we report on instructional trials of the ThinkerTools Inquiry Curriculum by 3 teachers in 12 urban classes in grades 7-9. The students' performance on both physics and inquiry assessments increased significantly from the pretests to the posttests. Their understanding of physics was assessed via problems that require the application of Newtonian principles (which are notoriously counterintuitive) to predicting how forces will affect the motions of various objects. Our results show that middle-school students who learn via this metacognitive, constructivist approach do better on such problems than high-school students taught using traditional approaches, regardless of their grade level (7-9) or whether their teacher had prior knowledge of the relevant physics.