Model-based reasoning is the process of reasoning with a set of ideas, specifically a scientific model. Emerging evidence suggests that model-based reasoning learning environments promote deep conceptual understanding in science, yet teachers are unfamiliar with these learning environments. Professional development is needed to assist teachers in establishing these learning environments, but much is unknown about the specific instructional features and mental processes necessary to successfully engage students in model-based reasoning. My research focused on indentifying and understanding the knowledge of practice generated by three teachers and a facilitator through professional experimentation with model-based reasoning. My qualitative research drew from a variety of data sets, such as teacher written responses, individual and focus group interviews, group dialogue, and presentations. Through multiple iterations of data coding, I triangulated and organized a set of findings for each domain of the Interconnected Model of Professional Growth (Clarke & Hollingsworth, 2002). In the Domain of Practice, I found that teachers reported engaging students in tasks centered in model development facilitated the use of collaborative groups, guiding questions, frequent reporting of ideas, attentiveness to student ideas, and the building of a consensus of ideas. In the Personal Domain, I found that teachers reported during model development lessons, teacher cognitive load increased through an elaborate process in which teachers accept student ideas and carefully process that information to formulate appropriate guiding questions. In the Domain of Consequence, I found that teachers reported model development lessons increased student intellectual engagement in learning and produced more rigorous student thinking, which correspondingly resulted in enhanced student learning and transfer of ideas. My primary analysis concluded that the curricular experimentation as instantiated through ISIM produced teacher change and professional growth in the three teacher participants of my study. Furthermore, the specific findings in each domain represented the knowledge-of-practice generated by the teachers through this experimentation with model-based reasoning. This knowledge of practice highlighted the critical importance of tasks centered in model development and the guiding question feedback cycle, described significant network chokepoints which could impinge upon successfully engaging students in model development, and suggested the professional development implications of these findings.