This mixed-method study examined the ways that principals and teachers coparticipated in discipline-focused online communities of practice designed to foster instructional improvement in two middle schools. Findings derived from interviews and content analysis of the online discussion threads revealed the emergence of a shared language about learning between principals and teachers, as well as how the scaffolding of knowledge development among those teachers was supported. Principal participation allowed teachers to gain insights about their principals' priorities, values, and beliefs about learning and was found to influence the quality of teacher-developed instructional units and teaching efficacy. Participative and directive leadership styles were represented by the principals, and their influence on the communication process and performance outcomes is discussed.