Districts across the country face significant demands to strengthen student learning districtwide, and many are turning to intermediary organizations to help them build their capacity for such demanding, large-scale work. However, how these "learning-support intermediary organizations" assist with these capacity-building efforts is little understood. This article reports data from a largely qualitative investigation into how one such intermediary organization, the Institute for Learning (IFL) at the University of Pittsburgh, partnered with multiple urban districts to help build district capacity for districtwide learning improvements. Our conceptual framework draws on sociocultural learning theory to identify key features of the IFL-district assistance relationships that seem associated with these outcomes. We utilized data from interviews, observations, document reviews, and focus groups conducted over a five-year period. Findings elaborate specific features of their assistance relationships--which we call adaptive assistance relationships--such as enabling particular forms of modeling, tools, and opportunities for rich dialogue. We conclude with implications for the research and practice of districtwide learning improvement efforts and the participation of intermediary organizations in the process. (Contains 2 footnotes.)