Our purpose in this article is to propose a comprehensive, empirically grounded theory of action for improving the quality of mathematics teaching at scale. In doing so, we summarize current research findings that can inform efforts to improve the quality of mathematics instruction on a large scale, and identify questions that are yet to be addressed. We draw on an ongoing collaboration with mathematics teachers, school leaders, and district leaders in four urban school districts in the US. Each year, we make recommendations to the districts based on the data we collect about how they might revise their strategies for instructional improvement to make them more effective. The provisional theory of action that we report encompasses curriculum materials and district-developed instructional guidance instruments, formal and job-embedded teacher professional development, teacher networks, mathematics coaches’ practices in providing job-embedded support for teachers’ learning, school instructional leadership in mathematics, and district leaders’ practices in supporting the development of school-level capacity for instructional improvement. In the final section of the article, we discuss areas in which additional studies are needed if research is to provide adequate guidance to school and district leaders who are attempting to support mathematics teachers’ development of equitable, inquiry oriented instructional practices.