This paper offers a conceptual model and rich narratives that describe the contexts in which teacher leadership emerges and the many ways that leadership is enacted. Drawing on qualitative case studies of seven Maine, USA schools, the authors found that teachers initiated their own professional learning efforts with the central goal of improving the conditions and outcomes of student learning. Teachers were leading through their strong commitment to continued learning, and by modeling a willingness to take risks, to collaborate and to question existing practices. Teachers often began with a focus on their own learning and classroom teaching and later moved into other leadership spheres where they collaborated with and influenced colleagues and other stakeholders on a wider scale. When they encountered conflict, teachers found they needed to build both interpersonal and intrapersonal awareness and skills. This study broadens conceptions of what constitutes teacher leadership and professional development. The work of teacher leaders results in teacher learning as well as improved student learning. The findings indicate that informal and formal, individual and collective teacher work all contribute to teachers' professional learning and significant change in schools. (Contains 1 table and 1 figure.)