Purpose: This article examines how and why four leadership functions are enacted in an elementary school, with a focus on hierarchical and "heterarchical" configurations of leadership. Research Design: The data were collected using an exploratory 2-year case study approach, and the data set comprises one-on-one interviews and relevant school documentation from school leaders and numeracy lead teachers engaged in reforming their mathematics teaching at an urban elementary school in a large New Zealand city. Four interrelated core functions of leadership are combined with configurations of leadership enactments to guide the analysis. Findings: The interplay between hierarchical and heterarchical enactments of leadership functions in hybrid forms appeared to strengthen the school's work toward embedding the reform. Results indicate that the lead teacher can effectively support reform goals when this role is shared with others and when one lead teacher also holds a designated leadership role in the school, such as that of assistant principal. A list of strategies that supported embedding a reform in this school was developed from the findings. Conclusions: Combining leadership functions and configurations into one analytical frame is useful in building an understanding of the complexities of school-based leadership by capturing leadership enactments across all members of a school staff, not just those in designated leadership positions. What appears to be important in promoting instructional improvement is hybrid patterns of leadership--the combination of hierarchical and heterarchical leadership enactments--rather than either of these on their own. (Contains 5 notes, 2 tables, and 2 figures.)