A core function of district leadership is providing instructional vision and support for school leaders and teachers. In this article, a Research-Practice Partnership focused on the improvement of elementary mathematics teaching and learning provides the context to study leadership development. We examine the process of one design team within the Research-Practice Partnership consisting of district leaders and university researchers using organizational learning. We found that as the team iteratively negotiated the design and implementation of a practical measure aimed at assessing and improving elementary mathematics instruction system-wide, the team's learning transformed the tools and routines of its work.