The gap in achievement in mathematics between at-risk students and their more advantaged counterparts is a persistent problem of the U.S. education system. Although some research-based curricula and pedagogy have demonstrated promise in supporting students from diverse backgrounds to develop conceptual understanding and procedural fluency in mathematics, scaling up instructional change across a district organization is a significant challenge. The Middle School Mathematics and the Institutional Setting of Teaching (MIST) Project is a research-practice partnership seeking to understand how large urban school districts can support the development of rigorous and equitable middle-school mathematics instruction at scale. This article enumerates the goals and design of this multiyear, multidistrict partnership, and describes one illustrative example of how our partnership activities informed and supported one district's efforts to improve mathematics instruction over multiple years. General recommendations for district--researcher partnerships are discussed.