Principals across the country are increasingly called on to support teacher learning and instruction as instructional leaders. As a result, districts are faced with the challenge of supporting principals to take on fundamentally new forms of leadership practice. However, there is limited understanding of (a) how principals might engage in learning those forms of practice, (b) the design of effective supports for principal learning, or (c) the processes by which effective supports might be designed. This study reports on a Design-Based Implementation Research approach to supporting principal learning in the context of a district effort for instructional improvement at scale. The context of the study involved an instructional improvement initiative across five elementary schools that included a vision of principal leadership that required principals to take on fundamentally new forms of practice. This vision of leadership was developed in relationship to particular goals for students' mathematical learning and, arising from those goals, new expectations for teachers' instructional practices. A Research-Practice Partnership (RPP) consisting of school district leaders and university-based researchers collaborated to support principals in developing these forms of practice and implementing the instructional improvement initiative. The study consists of three related analyses that examine the process of supporting principal learning from three different angles. The first analysis examines how principals engaged in learning new forms of leadership practice. I argue that principal learning was best conceptualized as an interwoven process of knowing, doing, and becoming, and that principals engaged in reconciling different expectations for the principalship in different ways. The second analysis examines the characteristics of the learning supports for principals that appeared to foster principals' becoming a new kind of leader in relation to particular goals for student and teacher learning. I propose a set of design principles that might be useful to others engaged in supporting principal learning of new ways of being a principal. Finally, the third analysis zooms in on one specific design process involve in the initiative: the decomposition of principal practice in effort to specify what mattered for principals to learn to enact. I examine the tensions within this design task and describe how the partnership responded to the tensions in productive ways. Taken together, these analyses contribute to understanding how to support principal to become fundamentally different school leaders. In particular, the analyses examine how supports might be designed to be responsive to local contexts and specific goals for principal, teacher, and student learning. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]