Purposes: This study investigates the role of formal and informal leaders in the diffusion of external reforms into schools and to teachers' practices. Formal leaders are designated by their roles in the formal organization of the school (e.g., principals, department chairs, and instructional coaches) and informal leaders refer to those who do not have any formal leadership roles but are nominated by other colleagues as influences on their instructional practices. In the context of implementing reading policies associated with No Child Left Behind (NCLB) 2001 legislation, this study aims to examine (a) how formal and informal leaders promote instructional changes through professional interactions with teachers and (b) which types of instructional practices are most responsive to which types of leaders. Research Methods: The authors analyze longitudinal data concerning both professional interactions about teaching reading and instructional practices of teachers and leaders in nine K-8 schools in a single state. Findings and Implications: Formal leaders convey influence on general teaching practices such as setting standards, selecting materials, and assessing students while informal leaders convey influence on specific pedagogical practices (e.g., the use of particular strategies for teaching basic reading skills). Findings contribute to the theoretical and methodological development of both distributed leadership and policy implementation within schools. Moreover, this study suggests the importance of and several strategies for developing a strong instructional leadership team that recognizes and supports the complementary influences of formal and informal leaders.