As science teachers major reforms in curriculum, instruction, and assessment, leaders at all levels must provide leadership and support that allows teachers to implement a new vision for science education within a challenging social, political, and economic climate. High school department chairs hold prime position to provide such leadership, but this role is under-researched and under-used within schools. Lack of time and authority and role conflict and ambiguity limit chairs effectiveness. Instructional leadership practice of science chairs represents a gap in the literature. Therefore, the purpose of this dissertation was to add to this limited understanding. Chapter 1 introduces the problem and lays out the theoretical framework, which draws on symbolic interactionism, narrative analysis, and Grounded Theory. Chapter 2 presents a review of historical literature on department chairsalong with reviews of literature on instructional leadership, science education leadership, and modeling within education researchand then concludes with a literature-based model of science instructional leadership. The remaining chapters present the empirical findings from three distinct phases of research and analysis. Chapter 3 presents the results of a descriptive survey of instructional leadership practices among science chairs across Georgia. Survey findings underscored the value of providing chairs with sufficient time and support to complete instructional duties. Further, a model developed from survey data provides a roadmap of how chairs build on supports and negotiate limitations to enact leadership within schools. Chapter 4 develops a comparative approach to narrative positioning analysis and presents findings from interviews with 11 exemplary science chairs and self-interviews with the author. Chairs leadership practice was strongly shaped by their school context and particularly by their positioning within the school leadership hierarchy. Chapter 5 builds on the previous analysis to develop a conceptual model of chairs instructional leadership practice that is jointly grounded in empirical data and existing literature. This analysis revealed that chairs more often positioned their practice within a discourse of school improvement than science education. Chapter 6 draws overarching conclusions and implications from this dissertation. Two important implications are that principals can manipulate the school leadership structure to empower chairs as instructional leaders and that chairs must act to educate teachers and administrators regarding the vision for science education reforms.