This proposal was submitted in response to EHR Core Research (ECR) program announcement NSF 15-509. The ECR program of fundamental research in STEM education provides funding in critical research areas that are essential, broad and enduring. EHR seeks proposals that will help synthesize, build and/or expand research foundations in the following focal areas: STEM learning, STEM learning environments, STEM workforce development, and broadening participation in STEM. The ECR program is distinguished by its emphasis on the accumulation of robust evidence to inform efforts to (a) understand, (b) build theory to explain, and (c) suggest interventions (and innovations) to address persistent challenges in STEM interest, education, learning, and participation. A team of investigators from the University of Arizona will conduct research on the teaching and learning of mathematical modeling in diverse elementary classrooms in Arizona and Washington. Mathematical modeling skills are foundational as they connect core mathematical ideas to real world experiences through analysis, representation, interpretation, and revision. Mathematical modeling also supports enduring understandings and practices in mathematics, such as problem solving and generalization. While mathematical modeling has a strong tradition in secondary education, the increased content of college and career readiness standards requires an introduction to modeling in elementary school. To address this need, the researchers will: (a) advance foundational knowledge on teaching and learning mathematical modeling in grades 3 through 5; (b) refine innovative professional development programs to increase teacher understanding and practice of mathematical modeling that draws on student experiences and relevant cultural and community contexts; and (c) develop a set of valid and reliable measures to evaluate teacher and student learning outcomes related to mathematical modeling. The project is funded by the EHR Core Research program, which supports fundamental research that advances the research literature on STEM learning, and has implications for education in both formal and informal settings. The project will be conducted as a mixed methods, multi-tiered design experiment. Across the three project years, researchers will iteratively develop and refine a professional development model with cohorts of teachers in Arizona and Washington who work in high poverty schools with culturally and linguistically diverse populations. The researchers will measure the potential for impact of the model on mathematics instruction and student learning. The proposed work to design mathematical modeling tasks is based on tenets of progressive formalization Realistic Mathematics Education (RME), which suggest that contexts matter for supporting student learning of mathematical modeling. The model integrates research on teaching and learning of mathematical modeling, and research on the role of culturally and community based funds of knowledge in teaching and learning mathematics. As students work on modeling tasks, they will produce artifacts that include evidence of reasoning and thinking. They will also be assessed by means of a Common Core aligned State-level Standardized Mathematics Assessment measure. Teachers will adapt mathematical modeling tasks, design lessons around those tasks, and iteratively refine a set of reusable tools for assessing students modeling activity. To measure impact on student learning, the researchers will use a two-group design to compare student learning in classrooms of training condition teachers with student learning in comparable, non-training condition classrooms. Since few teachers currently use mathematical modeling instruction in grades 3-5, a teacher comparison group to measure impact on teacher learning will not be employed. Rather, the researchers will use participating teachers as their own controls to examine changes over time in their understanding and practice.