Teachers are crucial to improving mathematics instruction in the US. Improvements in understanding teachers' knowledge about mathematics for teaching have led to improvements in teacher education and professional development. However, there are gaps in the understanding of mathematics knowledge for teaching, including how teachers reason about curriculum. Because curricular resources, including textbooks, continue to be important resources and tools for teachers, understanding how teachers reason about curriculum are important to improve teachers' knowledge and instructional practice. This collaborative project with the University of Nevada Las Vegas, Brigham Young University, University of Arkansas, and Grand Valley State University, will study middle grade mathematics teachers' curricular reasoning and develop a taxonomy that can be used by curriculum developers, teacher educators and professional developers to help in the development of productive curricular reasoning for teachers. This research is funded by the EHR Core Research Program, which emphasizes fundamental STEM education research that will generate foundational knowledge in the field. Investments are made in critical areas that are essential, broad and enduring: STEM learning and STEM learning environments, broadening participation in STEM, and STEM workforce development. <br/><br/>This three stage study will begin with adaptation, development, and pilots of the interview and observation protocols with eight teachers to verify that these instruments are valid and reliable. The second stage focuses on developing the curricular reasoning taxonomy by collecting analyzing interview and observation data from 12 grade 8 mathematics teachers in four states. These teachers will be using novel curriculum materials to teach geometry transformations during their regular classroom instruction. During this stage, the project will develop an initial taxonomy of teachers' curricular reasoning as they plan and enact these lessons. In the third stage, the researchers will verify and refine the curricular reasoning taxonomy by collecting interview and observation data from 20-40 grade 8 mathematics teachers in four states with a variety of teaching backgrounds.