We argue that learning about the nature and utility of scientific models and engaging in the process of creating and testing models should be a central focus of science education. To realize this vision, we created and evaluated the Model-Enhanced ThinkerTools (METT) Curriculum, which is an inquiry-oriented physics curriculum for middle school students in which they learn about the nature of scientific models and engage in the process of modeling. Key components of our approach include enabling students to create computer models that express their own theories of force and motion, evaluate their models using criteria such as accuracy and plausibility, and engage in discussions about models and the process of modeling. Curricular trials in four science classes of an urban middle school indicate that this approach can facilitate a significant improvement in students' understanding of modeling. Further analyses revealed that the approach was particularly successful in clarifying and broadening students' understanding of the nature and purpose of models. The METT Curriculum also led to significant improvements in inquiry skills and physics knowledge. Comparisons of METT students' performance with that of prior ThinkerTools students suggest that the acquisition of metamodeling knowledge contributed to these gains. In particular, METT students wrote significantly better conclusions on the inquiry test and performed better on some of the far-transfer problems on the physics test. Finally, correlational results, including significant correlations of pretest modeling and inquiry scores with posttest physics scores, suggests that developing knowledge of modeling and inquiry transfers to the learning of science content within such a curriculum. Taken together, the findings suggest that an emphasis on model-based inquiry, accompanied by the development of metamodeling knowledge, can facilitate learning science content while also developing students' understanding of the scientific enterprise.