We present a framework for considering principals' knowledge and actions to support high-quality instruction in a specific content area (mathematics). Using design research, we engaged principals in professional development and assessed principals' ability to identify aspects of high-quality mathematical tasks and instruction through pre-post task sort analyses and classroom video analyses. Significant differences occurred in principals' identification of high-quality mathematics tasks and instruction, students' thinking, and teachers' actions. Subsequent data identified changes in principals' feedback to mathematics teachers; however, this change was not sustained in following years. We hypothesize necessary conditions for supporting principals as instructional leaders in specific content areas.