This study elaborates the many ways that teachers lead work with colleagues to improve teaching and learning, and their understanding of their work as leadership. Through qualitative case studies of seven Maine schools and a review of the literature, the authors developed a conceptual model, Spheres of Teacher Leadership Action for Learning. They describe the various strategies teachers used to influence colleagues in direct and indirect ways, through formal and informal leadership. The authors discuss the importance of relationships, informal collaboration, trust and collegiality in supporting teachers' leadership development and school improvement. However, they also found teachers engaging in leadership to build these supportive conditions where they did not exist in schools. Teachers leading school improvement work were reluctant to see themselves as leaders, and rarely referred to themselves or others as "leaders". In fact, they viewed their informal and collaborative work as having greater impact on school improvement than formal efforts directed by school administrators. Yet, teachers did recognize the contributions and individual strengths that colleagues brought to their collective efforts. The authors suggest that in advancing the focus on school improvement and a shared accountability for the learning of all children, the term 'teacher leader' may be counterproductive.